Compound of formula (I) or a salt thereof: ** Formula ** wherein R1 is C1-10 alkyl optionally having 1 to 5 substituents selected from the group G1； C2-10 alkenyl optionally having 1 to 5 substituents selected from the group G1； C2-10 alkynyl having optionally 1 to 5 substituents selected from the group G1； -C1-10alkylene-X- (C1-10alkyl optionally having 1 to 5 substituents selected from the group G1)； R4； 5- or 6-membered heterocyclic group having 1-4 hetero atoms selected from oxygen and sulfur, and optionally having 1 to 5 substituents selected from the group G3； -C1-10alkylene； -C2-6-R4-alkenylene； -C1-10alkylene-X-R4； or -C1-10alkylene-X-C1-10alkylene-R4, or R1 forms, together with R21 or R22 in the pyridine ring attached thereto, a hydrocarbon ring fused with the pyridine ring, represented by any of the following Formulas (i) to (iv): ** Formula ** in which the hydrocarbon ring optionally has 1 to 4 substituents selected from the group consisting of C1-10 alkyl optionally having 1 to 5 substituents selected from the group G1, cycloalkyl optionally having 1 to 5 substituents selected from the group G2, -C1-10alkylene-R4 and -O-C1-10alkylene-R4, or the hydrocarbon ring optionally forms a spiro ring with C3-8 cycloalkane, R21 and R22 are the same or different and each is H； C1-10 alkyl optionally having 1 to 5 substituents selected from the group G1； -X-C1-10 alkyl； -X-halogen-C1-10 alkyl； cycloalkyl optionally having 1 to 5 substituents selected from the group G2； or -C1-10alkylene- (cycloalkyl having optionally 1 to 5 substituents selected from the group G2), R3 is C1-10 alkyl optionally having 1 to 5 substituents selected from the group G1； -C1-10alkylene-X- (C1-10alkyl optionally having 1 to 5 substituents selected from the group G1)； -C1-10alkylene-X- (C2-10 alkenyl having optionally 1 to 5 substituents selected from the group G1)； -C1-10alkylene- (cycloalkyl optionally having 1 to 5 substituents selected from the group G2)； -C1-10alkylene-X- (cycloalkyl opt
