A therapeutic agent for prostate cancer and malignant lymphoma containing FK228 or a salt thereof as an active ingredient, and a method for evaluating an antitumor effect of a histone deacetylase inhibitor which includes at least a step of treating a test cell with a histone deacetylase inhibitor, a step of measuring change in the expression amount of a specific gene in the test cell before and, after the treatment with the inhibitor, comparing the both expression amounts.