Actually this isn't the poison gas factory itself, but rather the power plant that powered the poison gas production facility. Most of the factory was destroyed in 1945.
It is located on the tiny island of Okunoshima just off the coast of Hiroshima Prefecture in the Seto Inland Sea.
From 1929 until 1945 the Japanese government maintained a top-secret installation producing more than 6,000 tons of Mustard Gas which was used in their campaigns in China.
There are still a couple of other structures left on the island but I did not have time to visit them.
There is a very good little museum on the subject on the island.
I wrote a guide to Okunoshima which can be found here