did a quick google search and found an ashtray with a similar glaze also marked 'japan'. it claims to have been produced during 50's to 60's. I do know that following WW2 the US helped Japan establish factories that would produce consumer goods in an effort to help re-establish an economy and put their people to work. For a while (1945-1952) goods were marked 'occupied Japan'. your ashtray is certainly post-1952. my guess is that it probably dates to late 50's