Abstract: The present invention provides a novel compound having an excellent PDE4 inhibitory activity and TNF-? production-suppressing activity, and also provides a preventive and therapeutic agent for atopic dermatitis or the like. The present invention includes a novel phthalazinone derivative of the following general structural formula [1] or a pharmaceutically acceptable salt thereof, and a pharmaceutical composition comprising it as an active ingredient. In the structural formula [1], the partial structure bridging the 6-position and the 7-position represents a single bond or a double bond; R1 and R2 are the same or different and each represents alkyl or the like; Y represents phenylene or the like; Z represents alkylene or the like; and R3 represents a mono- to tri-cyclic saturated or unsaturated cyclic amino group or the like optionally substituted with R31 and R32, wherein R31 and R32 represent alkyl or the like.