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.