A novel immunosuppressive agent, more specifically an immunosuppressive agent comprising, as an active ingredient, a heterocyclic compound represented by the general formula (I) (wherein X or other variables are as defined in the specification) or a pharmaceutically acceptable salt thereof; a novel heterocyclic compound represented by the general formula (II) (wherein X or other variables are as defined in the specification) or a pharmaceutically acceptable salt thereof; and use of the compound or salt as an antitumor agent.