A compound represented by a formula or a salt thereof: wherein A 1 represents CR 1 or a nitrogen atom; A 2 represents CR 2 or a nitrogen atom; A 3 represents CR 3 or a nitrogen atom; provided that two or more of A 1 to A 3 do not represent nitrogen atoms at the same time; R 1, R 2 and R 3 each independently represent a hydrogen atom, a substituted or unsubstituted C 1-6 alkyl group, a hydroxyl group, a substituted or unsubstituted C 1-6 alkoxy group, a nitro group, a cyano group, a substituted or unsubstituted amino group or a halogeno group; B 1 represents a nitrogen atom; R 4 represents a substituted or unsubstituted C 1-6 alkyl group, a substituted or unsubstituted C 2-6 alkenyl group, a substituted or unsubstituted C 2-6 alkynyl group, a hydroxyl group, a substituted or unsubstituted C 1-6 alkoxy group, a formyl group, a substituted or unsubstituted C 1-6 alkylcarbonyl group, a substituted or unsubstituted C 1-6 alkoxycarbonyl group, a substituted or unsubstituted C 1-6 alkylthio group, a substituted or unsubstituted C 1-6 alkylsulfinyl group, a substituted or unsubstituted C 1-6 alkylsulfonyl group, a substituted or unsubstituted C 1-6 alkylsulfonyloxy group, a substituted or unsubstituted C 3-8 cycloalkyl group, a substituted or unsubstituted 5- to 6-membered heteroaryl group, a substituted or unsubstituted amino group, a substituted or unsubstituted aminocarbonyl group, or a group represented by -N=CR a R b, wherein R a represents a hydrogen atom or a C 1-6 alkyl group, and R b represents a C 1-6 alkyl group; R 5 represents a substituted or unsubstituted C 1-6 alkylthio group, a substituted or unsubstituted C 1-6 alkylsulfinyl group, or a substituted or unsubstituted C 1-6 alkylsulfonyl group; R 6 represents a substituted or unsubstituted 5- to 6-membered heteroaryl group, a substituted or unsubstituted amino group represented by -NR g R h, or a substituted or unsubstituted aminocarbonyl group, wherein R g and R h each independently represents a hydrogen atom, a C 1-6 alkyl group, a formyl group, a C 1-6 alkylcarbonyl group, a C 1-6 haloalkylcarbonyl group, a C 3-8 cycloalkylcarbonyl group, a C 1-6 alkoxycarbonyl group, a substituted or unsubstituted amidino group, a substituted or unsubstituted aminocarbonyl group, a substituted or unsubstituted aminosulfonyl group, or a C 1-6 haloalkylsulfonyl group. A 1 represents CR 1 or a nitrogen atom; A 2 represents CR 2 or a nitrogen atom; A 3 represents CR 3 or a nitrogen atom; provided that two or more of A 1 to A 3 do not represent nitrogen atoms at the same time; R 1, R 2 and R 3 each independently represent a hydrogen atom, a substituted or unsubstituted C 1-6 alkyl group, a hydroxyl group, a substituted or unsubstituted C 1-6 alkoxy group, a nitro group, a cyano group, a substituted or unsubstituted amino group or a halogeno group; B 1 represents a nitrogen atom; R 4 represents a substituted or unsubstituted C 1-6 alkyl group, a substituted or unsubstituted C 2-6 alkenyl group, a substituted or unsubstituted C 2-6 alkynyl group, a hydroxyl group, a substituted or unsubstituted C 1-6 alkoxy group, a formyl group, a substituted or unsubstituted C 1-6 alkylcarbonyl group, a substituted or unsubstituted C 1-6 alkoxycarbonyl group, a substituted or unsubstituted C 1-6 alkylthio group, a substituted or unsubstituted C 1-6 alkylsulfinyl group, a substituted or unsubstituted C 1-6 alkylsulfonyl group, a substituted or unsubstituted C 1-6 alkylsulfonyloxy group, a substituted or unsubstituted C 3-8 cycloalkyl group, a substituted or unsubstituted 5- to 6-membered heteroaryl group, a substituted or unsubstituted amino group, a substituted or unsubstituted aminocarbonyl group, or a group represented by -N=CR a R b, wherein R a represents a hydrogen atom or a C 1-6 alkyl group, and R b represents a C 1-6 alkyl group; R 5 represents a substituted or unsubstituted C 1-6 alkylthio group, a substituted or unsubstituted C 1-6 alkylsulfinyl group, or a substituted or unsubstituted C 1-6 alkylsulfonyl group; R 6 represents a substituted or unsubstituted 5- to 6-membered heteroaryl group, a substituted or unsubstituted amino group represented by -NR g R h, or a substituted or unsubstituted aminocarbonyl group, wherein R g and R h each independently represents a hydrogen atom, a C 1-6 alkyl group, a formyl group, a C 1-6 alkylcarbonyl group, a C 1-6 haloalkylcarbonyl group, a C 3-8 cycloalkylcarbonyl group, a C 1-6 alkoxycarbonyl group, a substituted or unsubstituted amidino group, a substituted or unsubstituted aminocarbonyl group, a substituted or unsubstituted aminosulfonyl group, or a C 1-6 haloalkylsulfonyl group.