The present invention relates to: a novel biomarker for diagnosing a cancer, containing MED30; a composition for diagnosing a cancer; a kit for diagnosing a cancer; and a method for diagnosing a cancer using the same. In addition, the present invention relates to a composition for preventing or treating a cancer, containing an MED30 inhibitor as an active ingredient. The MED30, according to the present invention, is significantly expressed in cancer tissues, and thus can be effectively used for diagnosing a cancer. In addition, the MED30 inhibitor can inhibit cancer proliferation, migration, and invasion, and thus it is expected that the MED30 inhibitor could be utilized as a novel anticancer drug.