Compound of formula (I) or salt thereof: ** Formula ** wherein R1 is hydrogen, halogen-C1-6 alkyl, C1-6 alkyl, or -O-C1-6 alkyl, R2 is a group of formula (II): ** Formula ** R3 is C1-6 alkylene (cycloalkyl which may be substituted with halogen or -O-C1-6 alkyl)； C1-6 alkylene-saturated hetero-ring containing oxygen； cycloalkyl which may be substituted with halogen or -C 1-6 alkyl； a saturated hetero ring containing oxygen； or a monocyclic saturated nitrogen-containing hetero ring which may be substituted with C1-6 alkyl, C1-6-aryl alkylene or -CO-C1-6 alkylene-C1-6 alkyl, R4, R5 and R6 are hydrogen, Ra and Rb are they combine with the adjacent nitrogen atom to form a monocyclic nitrogen-containing hetero ring, which may be substituted with a group selected from: -OH； halogen-C1-6 alkyl； -O-C1-6 alkyl which may be substituted with 1 to 3 groups selected from the group consisting of halogen, halogen-C1-6 alkyl and cycloalkyl； aryl that may be substituted with a group selected from a group G1； a hetero ring that may be substituted with a group selected from a group G2； C1-6-O-cycloalkyl alkylene； -O-cycloalkyl； -O- (hetero ring that may be substituted with a group selected from a group G2)； C1-6 alkyl which may be substituted with one or more groups selected from the group consisting of halogen, halogen-C1-6 alkyl, -O-cycloalkyl, -O-C1-6 alkyl and -O-halogen-C1-6 alkyl ； and C 1-6 alkylene-C 1-6 alkyl which may be substituted with one or more groups selected from the group consisting of halogen, halogen-C 1-6 alkyl and cycloalkyl, the group G 1 consists of halogen, C 1-6 alkyl, halogen-C1-6 alkyl, -OH, -O-C1-6 alkyl, -O-heteroanillo, -O-C1-6 alkylene, -O-C1-6 alkylene-heteroanillo, -O-halogen-C1 alkyl -6, -N (C1-6 alkyl) 2, C1-6 alkylene-N (C1-6 alkyl) 2, C1-6 alkylene heteroaryl, aryl which may be substituted with C1-6 alkyl, a hetero ring which may be substituted with C1-6 alkyl, -COOH, -CO-O-C1-6 alkyl, -CO-O-C1-6 alkylene-C1-6 alkyl, -CO-O-C1-6-aryl alkylene, -CO-
