Provided is a pest control agent containing a fused ring pyrimidine compound represented by the following formula (I) or a salt thereof as an effective component. In formula (I), A represents a nitrogen atom or C&mdash；R4, B represents a nitrogen atom or C&mdash；R5, D represents a nitrogen atom or C&mdash；R6, X represents an oxygen atom, N&mdash；R7 or the like, each of R1, R2, R4 and R6 independently represents a hydrogen atom or the like, each of R3a and R3b independently represents a hydrogen atom or the like, R5 represents an alkylthio group or the like, R7 represents an alkoxy group or the like, and Ht represents a pyridyl group that may be substituted, or the like.
