The purpose of the present invention is to provide a novel tumor treatment agent. In GREB1-positive tumors that do not exhibit sex hormone sensitivity, GREB1 is a downstream target gene of the Wnt/β-catenin signal. Through the use as an active ingredient of a substance that can inhibit GREB1 expression, the growth of GREB1-positive tumor cells that do not exhibit sex hormone sensitivity can be effectively inhibited and such a tumor can be treated.