The question is somewhat ambiguous. Japan started fighting in China in 1933, so to be pedantic, we'd look at a 1930 census. From an American viewpoint, the war didn't start until the Japanese attack on Pearl Harbor, 7 December 1941.
Hiroshima had a population of 344,000 according to the 1940 census. I can't find anything between 1910 and 1933, but the 1910 census figure was 143,000.