An object of the present invention is to provide methods for diagnosing and/or treating tumors using HERV-H env gene or HERV-H env protein. Specifically, tumors are diagnosed by detecting expression of HERV-H env gene； and agents for detecting the expression are used as diagnosing agents. Further, tumors are treated by inhibiting function of HERV-H env gene； and agents for inhibiting the function are used as antitumor agents. Furthermore, tumors are treated by administering a peptide having a certain sequence of HERV-H env protein and the like； and the peptide is used as a cancer vaccine.
