Around 140,000 people perished instantly in the searing heat or from radiation in the days and months after a US plane unleashed the deadliest weapon ever used and ushered in the nuclear age.
Nearly seven decades later, Tsuboi, one of a dwindling number of survivors of the first ever atomic attack, is raising his voice against nuclear power in a country still reeling from the tsunami-sparked catastrophe of March 2011.
“In terms of being nuclear victims, we are the same,” Tsuboi, 87, said of those affected by the Fukushima crisis.
He was on his way to university when the bomb exploded over Hiroshima in a flash of blinding light and intense heat on August 6, 1945.
As well as his burns, Tsuboi has also suffered intestinal cancer that may be linked to radiation exposure, and says he sees little difference in the dangers posed by atomic weapons and atomic power.
“Nuclear technology is beyond human wisdom… I still want to see a nuclear-free world while I’m alive,” he said.
His appeal comes as a bitter debate swirls over the future of Japan’s 50 remaining reactors, which once met around a third of the country’s electricity needs, but which were shuttered following the meltdowns at Fukushima.
Fears of electricity shortages have led to the government ordering restarts at two reactors, despite an increasingly vocal anti-nuclear movement in a country largely unused to public protest.
Those who experienced the World War II bombing in Hiroshima and a similar attack on the port city of Nagasaki three days later, said television images of the Fukushima crisis brought back terrible memories.
“The TV reminded me of the dreadful scenes,” said a sobbing Misako Katani, 82, one of just a few living victims who survived both bombings.
No one is officially recorded as having died as a result of the Fukushima disaster, but many who fled the area and those who remain, including workers decommissioning the crippled plant, worry about the long-term effects.
The quake-sparked tsunami knocked out the reactors’ cooling systems, causing meltdowns that spread radiation over a large area and forced thousands to evacuate.
Scientists have warned it could be decades before it is safe for some people to return to their homes.
Sachiko Sato, a Fukushima evacuee who was among tens of thousands of people attending an annual Hiroshima commemoration event on Monday, said: “I think we can share the same sadness with people in Hiroshima and Nagasaki.
“In my mind, Fukushima is like a third nuclear victim following Hiroshima and Nagasaki.”
Atom bomb survivor Toshiyuki Mimaki, 70, added: “We want to work together with people in Fukushima and join our voices in calling for no more nuclear victims.”
But for some ageing victims, there are few parallels between 1945 and 2011.
“There is nothing to compare to what I experienced,” said Shigeji Yonekura, 79, who was at Hiroshima.
“The atomic bomb was dropped in war and no one helped us, while the Fukushima accident occurred in peace time and a lot of people offered help.”
Supporters of the nuclear attacks on Japan maintain they brought a quick end to the war by speeding up Tokyo’s surrender, preventing millions more casualties from a land invasion planned for later in the year.
Despite his own experience, Yonekura is resigned to the possibility that resource-poor Japan may not be able to abandon atomic power altogether.
“Nuclear power may be a necessary evil,” he said.
But Miyako Jodai, a survivor of the Nagasaki bombing, which killed 70,000 people, said the Fukushima accident and the way the crisis was managed had turned her against atomic energy.
Several reports on the accident have heaped criticism on government and plant officials, with one parliamentary probe calling Fukushima a “man-made disaster”.
“I had been convinced that peaceful use of nuclear power should be accepted because reactors were safe,” said Jodai.
“But after seeing the accident and the government’s handling of the aftermath, I felt I was betrayed.”