A compound represented by a formula (I) or a salt thereof: wherein A1 represents CR1 or a nitrogen atom; A2 represents CR2 or a nitrogen atom; A3 represents CR3 or a nitrogen atom, provided that two or more of A1 to A3 do not represent nitrogen atoms at the same time; R1, R2 and R3 each independently represent a hydrogen atom or the like; B1 represents CH or a nitrogen atom; R4 represents a substituted or unsubstituted C1-6 alkyl group or the like; R5 represents a substituted or unsubstituted C1-6 alkylthio group or the like; and R6 represents a substituted or unsubstituted C1-6 alkyl group or the like.