Isoxazole derivative and agricultural/horticultural plant disease control agent

Provided is a superior agricultural/horticultural plant disease control agent, and a production intermediate thereof. Provided are an agricultural/horticultural plant disease control agent, and a production intermediate thereof, said agent containing, as an active ingredient thereof, an isoxazole derivative represented by general formula (I) (where, X is a C 1 to C 6 alkyl group, a C 1 to C 6 haloalkyl group or the like, Y 1 is a hydroxyl group, a C 1 to C 6 alkoxy group or the like, Y 2 is a hydrogen atom or the like, Q 1 is a phenyl group, a pyrazolyl group or the like, Q 2 is a phenyl group, a pyridyl group, a pyrimidinyl group or the like, Q 3 is a pyridyl group, a pyrimidinyl group or the like, and Q 1 , Q 2 and Q 3 may each be substituted by a halogon atom or the like), or an agrochemically acceptable salt thereof.