Hidden among palms and bracken are concrete caves built into the hillside, containing circular cradles of crumbling cement; on the far side of the island is the gaping hulk of an old power station, populated by a colony of albino rabbits.
"The A-bomb victims talk about how terrible their experiences were, but when people pray for peace, and try to understand the nature of the war, there are two places which they should visit," said Hatsuichi Murakami, the curator of Okunojima's museum.
"The first is the Peace Museum in Hiroshima. The second is here. Hiroshima represents Japan's position as victim. Okunojima shows us about Japan the aggressor."
The shocked Japanese military used the euphemism "special bomb" for the device which destroyed Hiroshima, and which is being exhaustively commemorated in rallies, exhibitions and this Sunday's climactic ceremony in the city's famous Peace Park.
But the Japanese are seldom reminded of their country's own "special" weapons. Between 1929 and 1945, a huge military complex on Okunojima produced 1,200 tons of half a dozen different toxic gases every year. They were used more than 2,000 times in Japan's 14-year war in China, and killed or injured as many as 80,000 enemy soldiers and civilians, not counting the many Japanese workers killed or maimed by the poison, sometimes decades later. Those concrete cradles once bore huge metal tanks of mustard gas; the rabbits in the power station are the descendants of laboratory animals used to test its lethal properties.
Mr Murakami, curator of the Okunojima Toxic Gas Museum, first came here in 1939, at the age of 14. When he was a boy on the mainland, the military drills at school were enlivened by the use of smoke bombs. These, he was told, came from the island, a few miles off shore. "My image of Okunojima," he said, "was of a very inspiring, mysterious, heroic place."
Japan had been experimenting with toxic gas since soon after the First World War. After the great Tokyo earthquake in 1923, production was transferred to the island.
As well as mustard gases, it produced phosgene, chlorine, cyanide and tear gas. The museum displays the tanks and filtration columns, and the all-over suits and gas masks, worn by horses as well as workers.
"The most impressive thing about the place," said Mr Murakami, "was what we were told about the purpose of the gas. It wasn't intended to kill the enemy, but just to paralyse him and sap his fighting spirit. It was a just and moral undertaking. But very soon I realised that something was wrong."
The mustard gas leaked through the rubber uniforms and gas masks, and it was common to see workers with blistered skin and lung diseases. The hydrochloric acid used as a reagent caused eye injuries, and years later workers suffered conjunctivitis, pleurisy, bronchitis and pneumonia.
Okunojima was not marked on Japanese maps and, even after the war, its secret was little known or talked about. In 1945 the complex was demolished and detoxified by the United States military.
Hundreds of tons of gas were loaded onto a freighter which was sunk far out in the Pac- ific. Until the Seventies, the Japanese government refused to acknowledge the plant's existence, but eventually pen- sions were paid to surviving workers.
The island's grimmest legacy is only now being tackled. In China, 2 million poison gas shells remain in underground dumps; earlier this year Japanese officials visited the sites and agreed to pay for their disposal.
Further burial sites have been identified a few hundred yards from Mr Murakami's museum, and even below a residential area in Hiroshima itself.
"We remember the atom bomb, and the agony the Americans caused," he said. "But on Okunojima many people suffered because of their own country. Do we remember that we suffered, or that we caused suffering ourselves?"