Kodaira was born in Tochigi prefecture on January 28, 1905. He joined the navy and took part in the Japanese assault on Jinan in 1928, where by his own later account he killed civilians, including a pregnant woman, and was decorated on his return. In 1932 he beat his father-in-law to death with an iron bar in a dispute and served a prison sentence, being released in 1940.
Between May 1945 and August 1946 he raped and killed seven women in Tokyo and Tochigi. Occupied Japan was starving, and he approached young women with offers of food, or of work at the naval supply depot where he was employed, and took them to bombed-out ground. He strangled them, and in several cases struck them with a hammer or an iron bar; some of the bodies were left where they were assaulted and some were buried. Higher totals up to ten circulate and rest on his own statements.
He was caught because a victim's family pursued the case themselves. Ryuko Midorikawa disappeared in July 1946 after leaving with him, her body was found in Shiba park, and her father gave police the name. He was arrested in August 1946.
He was tried and sentenced to death, and hanged at Miyagi prison on October 5, 1949. The case is one of the best known of the immediate post-war period in Japan, both for the number and for what the trial showed about the earlier killings in China, which had gone not merely unpunished but rewarded.