PROBLEM TO BE SOLVED: To provide a novel nitrogen-containing heterocyclic compound and an agricultural/horticultural fungicide containing the compound.SOLUTION: The nitrogen-containing heterocyclic compound is represented by formula (I), and the agricultural/horticultural fungicide contains the compound. (In the formula, Rand Reach represent an unsubstituted or substituted C1-6 alkyl group or the like; Rrepresents an unsubstituted or substituted C1-6 alkyl group or the like; Rrepresents an unsubstituted or substituted C1-6 alkyl group or the like; Band Beach represent a carbon atom or a nitrogen atom; and X represents an oxygen atom, a sulfur atom, or the like.)【課題】新規な含窒素ヘテロ環化合物、及び該化合物を含有する農園芸用殺菌剤の提供。【解決手段】式(I)で表される含窒素ヘテロ環化合物、及び該化合物を含有する農園芸用殺菌剤。(式中、R1及びR2は、無置換または置換基を有するC1~6アルキル基など、R3は、無置換または置換基を有するC1~6アルキル基など、R4は、無置換または置換基を有するC1~6アルキル基など、B2及びB3は、炭素原子または窒素原子、Xは、酸素原子、硫黄原子などを示す。)【選択図】なし