The present invention provides a
phthalazine compound as a therapeutic agent for
erectile dysfunction represented by the following formula, a pharmacologically acceptable salt thereof or a
hydrate thereof:wherein R1 and R2 are the same as or different from each other and represent a
halogen atom, a C1 to C4
alkyl group which may be substituted with a
halogen atom, a C1 to C4
alkoxy group which may be substituted with a
halogen atom or a cyano group; X represents a cyano group, a nitro group, a halogen atom, a hydroxyimino group which may be substituted or a heteroaryl group which may be substituted; Y represents a heteroaryl group, an
aryl group which may be substituted, an alkynyl group which may substituted, an alkenyl group, an
alkyl group, an optionally substituted saturated or unsaturated 4- to 8-membered amine ring, and the cyclic amine compound is a monocyclic compound, bicyclic compound or a
spiro compound; l is an integer of 1 to 3; provided that the case where l is 1 or 2, X is a cyano group, a nitro group or a
chlorine atom, R1 is a
chlorine atom, R2 is a methoxy group and Y is a 5- or 6-membered amine ring substituted with a hydroxyl group is excluded.