A compound represented by formula (I) or a salt thereof. In formula (I): 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 independently represent a hydrogen atom, etc.； B1 represents CH or a nitrogen atom； R4 represents a substituted or unsubstituted C1-6 alkyl group, etc.； R5 represents a substituted or unsubstituted C1-6 alkylthio group, etc.； and R6 represents a substituted or unsubstituted C1-6 alkyl group, etc.
