The present invention provides an anti-tumor agent and a suppressor for suppressing malignant transformation such as acquisition of metastatic ability or acquisition of apoptosis resistance of a tumor cell. The present invention provides a tumor cell malignant transformation suppressor, including a substance suppressing or inhibiting Zic5 gene functions as an active ingredient to suppress or inhibit acquisition of metastatic ability or acquisition of apoptosis resistance of a tumor cell; a method of suppressing tumor cell malignant transformation of animals other than humans, including suppressing or inhibiting Zic5 gene functions to suppress or inhibit acquisition of metastatic ability or acquisition of apoptosis resistance of a tumor cell; an anti-tumor agent including a substance suppressing or inhibiting Zic5 gene functions as an active ingredient, in which the agent is used in prostate cancer therapy; and a marker for tumor cell malignancy consisting of the expression level of Zic5 gene.