Over the course of his career Hayashi has researched Japanese history during World War II.
He has researched the Sook Ching massacres in Singapore,[1] discovered that US war crimes trials began on Guam before the Japanese surrender in 1945.
He argued that Okinawa residents' mass suicides were effectively under the control of the Japanese military, even if no direct orders were given.
[3] Hayashi has discovered documentary proof of Japanese military's wide and common involvement in the forced sexual slavery of the comfort women on Java in Indonesia, in Lansong Province in Vietnam, and in Guilin City in China.
[6] On April 7, 2014, Hayashi found evidence of the forced sexual slavery on Bali in Indonesia.