Did I suffer radiation exposure?
If so, to what extent?
How has it affected my health?
And most painfully of all—why don’t I know the truth about what happened to me?
These are the questions that plague Kazuma Masumoto, a former crew member from Kochi City, who, more than 60 years ago, worked aboard the tuna fishing vessel Hime-maru. During the height of the Cold War, the United States conducted repeated nuclear tests in the Pacific, affecting nearly 1,000 Japanese fishing boats operating in the surrounding waters—including Hime-maru.
Only in recent years has the full extent of these nuclear tests begun to come to light. In court, the Japanese government has been forced to acknowledge cases of radiation exposure among former fishermen. Yet justice remains elusive. A radiation exposure study conducted by Japan’s Ministry of Health, Labour and Welfare was marred by errors; the study’s own lead researcher admitted to a margin of error of “two to three times,” prompting former crew members to question whether their exposure had been deliberately downplayed.
What is the real truth? For 82-year-old Masumoto, uncovering that truth has become a life mission.
( written by Yasuhito Sasajima, 2019 )
On January 22 of this year, Masumoto stood in the Takamatsu High Court for the first hearing of his appeal. The courthouse was more imposing than the one where his first trial had taken place, and the courtroom itself was slightly larger. Journalists and supporters filled the gallery. “I felt a sense of resolve,” he later reflected.
When he first filed suit in 2016, he never imagined he would stand in court himself. His case revolves around events that took place more than six decades ago: the nuclear tests and his own exposure. Yet his opponent in the lawsuit is not the country responsible for those tests—the United States—but rather his own government, Japan.
Masumoto was born on the Korean Peninsula under Japanese rule and, after World War II, returned to Kochi Prefecture, a fishing community, with his family. At 17, he joined the tuna fishing industry to help support them. His first assignment was aboard Hime-maru.
"Life on a tuna fishing boat was brutal," he recalls. "People died. If you got injured, they’d tell you to rub grease on it and keep going. We were treated like disposable labor. Even so, men lined up to work. Some were as young as 15. Why? Because they needed to eat. Because they needed to provide for their families.”
Masumoto later transitioned to a commercial vessel after getting married. He has battled numerous illnesses, but for the most part, lived what he describes as an “ordinary life.”
During his time on Hime-maru, the United States repeatedly conducted nuclear tests in the Pacific. On March 1, 1954, one of those tests—a hydrogen bomb detonation at Bikini Atoll—exposed the Daigo Fukuryu Maru, a Japanese tuna fishing boat from Yaizu, Shizuoka Prefecture, to nuclear fallout.
The bomb was 1,000 times more powerful than the atomic bomb dropped on Hiroshima. Fallout rained down over 160 kilometers away, irradiating the Fukuryu Maru’s crew. The ship’s radio operator, Aikichi Kuboyama, died six months later. The contamination from these nuclear tests was so widespread that, by the end of 1954, nearly 1,000 fishing vessels had unknowingly brought radioactive fish back to Japan, causing national panic over “nuclear-tainted tuna.”
For Masumoto, the past came rushing back in March 2014. A local newspaper published an article about the nuclear tests, featuring testimonies from former fishermen:
"We caught beautiful tuna, only to be forced to dump them into the sea after testing."
"In the middle of the night, the sky suddenly lit up like daytime."
"Ash-like particles rained down on us."
Yet in January 1955, less than a year after the Bikini Atoll disaster, the Japanese government signed an agreement with the United States, declaring the matter “fully resolved.” In exchange for a $2 million “compassionate payment,” Japan relinquished all rights to seek further compensation for its affected citizens. U.S. nuclear tests continued, but portside radiation screenings were abandoned.
Masumoto realized that his own experience aboard Hime-maru overlapped with the testimonies he was reading. More than 60 years had passed, yet the memories had never fully faded.
Determined to uncover the truth, he set out to investigate. He tracked down former Hime-maru crewmates, researched their medical histories, and spoke with bereaved families. He even located his former captain, who admitted to witnessing the nuclear blast. In his own town, he discovered another fisherman who had worked in the same waters and died in his twenties. His brother recalled, “In his final moments, blood poured from every opening in his body.”
Masumoto’s wife, Miho, has meticulously archived these findings, their collection of evidence growing thicker by the year.
In 2015, as renewed attention turned to the Bikini Atoll nuclear tests, the Ministry of Health, Labour and Welfare launched a study to assess the radiation exposure of former fishermen. Many hoped that, at last, the government was ready to listen.
Yet when the study’s report was published in 2016, it was revealed that not a single affected fisherman had been interviewed.
The report estimated that the maximum radiation dose received by the fishermen was 1.12 millisieverts, an amount deemed too low to cause health effects. However, the study’s lead researcher later admitted that the actual radiation levels could have been two to three times higher—acknowledging the possibility that the report understated the true extent of exposure.
For comparison, when the same calculation method was applied to the Daigo Fukuryu Maru, it estimated an exposure of 1,300 millisieverts. In reality, direct measurements taken from the ship’s crew showed radiation levels of up to 7,000 millisieverts, with an average of 3,000. The government had deliberately left this information out of the report.
Crucially, the study failed to consider internal radiation exposure—such as consuming contaminated fish or using seawater for daily needs.
Realizing that the government would never admit the full truth, Masumoto resolved to prove his own exposure. He gathered official documents, including ship records proving that Hime-maru had been operating in contaminated waters. He also obtained the death certificate of a former crewmate, who had died of acute monocytic leukemia—one of the few illnesses officially recognized as radiation-related in Japan’s worker compensation system.
Masumoto himself has suffered from multiple illnesses, including prostate cancer. A doctor who once assessed the Daigo Fukuryu Maru crew confirmed that several of Masumoto’s conditions could be linked to radiation exposure. His wife, a retired nurse, helped preserve his medical records, which became crucial evidence in his legal battle.
At 82, Masumoto is in a race against time.
He has watched as fellow plaintiffs passed away before they could see justice. Since the lawsuit began, the number of surviving plaintiffs has dwindled from 45 to 29. “I just want this to be resolved before it’s too late,” he says.
“People need to know why we were on those fishing boats. We worked because we had no choice. We paid taxes, we fulfilled our duties as citizens. But the government has failed in its duty to us. They settled this behind closed doors, trampling over us in the process. Many of my crewmates died without knowing the truth. I want to clear their names.
