A compound represented by the formula (I'): wherein: ring A' is (i) a benzene, (ii) a piperidine, or (iii) a tetrahydropyridine which can be substituted by 1 to 4 substituents selected from a halogen atom and a C 1-4 alkoxy group; ring B is selected from the group consisting of C 3-12 cycloalkyl, dihydropyranyl, tetrahydropyranyl, azetidinyl, pyrrolidinyl, piperidinyl, imidazolyl, isoxazolyl, pyrazolyl, dihydropyrazolyl, pyridyl, pyrrolyl, dihydropyrrolyl, phenyl, morpholinyl, thiazolyl, oxazolidinyl, imidazolidinyl, oxaazabicyclooctyl, and oxazepanyl, which may optionally be substituted by 1 to 4 substituents selected from a halogen atom, a hydroxy group, a cyano group, an oxo group, a C 1-10 alkoxy-carbonyl group, a C 1-10 alkoxy group which can be substituted by halogen, provided that a group represented by the formula where p is 1 to 4, which may be optionally substituted, is excluded; R 1 is a C 1-4 alkoxy group which can be substituted by one or more substituents selected from a C 1-4 alkoxy group, a halogen atom and a C 3-7 cycloalkyl group, wherein the optionally substituted C 1-4 alkoxy is radiolabeled with 11 C or 18 F; R 2 is a halogen atom, a C 1-10 alkyl group which can be substituted or a C 6-14 aryl group which can be substituted; R 3 is a halogen atom, a C 1-10 alkyl group which can be substituted or a C 1-10 alkoxy group which can be substituted; m is 0 or 1; and n is 0 to 3; provided that 1-[4-(3,5-dimethylisoxazol-4-yl)-2-fluorophenyl]-5- 11 C-methoxy-3-(1-phenyl-1H-pyrazol-5-yl)pyridazin-4(1H-one and 1-(2-fluoro-4-(morpholin-4-yl)phenyl)-5- 11 C-methoxy-3-(1-phenyl-1H-pyrazol-5-yl)-pyridazin-4 -one are excluded; or a salt thereof. ring A' is (i) a benzene, (ii) a piperidine, or (iii) a tetrahydropyridine which can be substituted by 1 to 4 substituents selected from a halogen atom and a C 1-4 alkoxy group; (i) a benzene, (ii) a piperidine, or (iii) a tetrahydropyridine which can be substituted by 1 to 4 substituents selected from a halogen atom and a C 1-4 alkoxy group; a halogen atom and a C 1-4 alkoxy group; ring B is selected from the group consisting of C 3-12 cycloalkyl, dihydropyranyl, tetrahydropyranyl, azetidinyl, pyrrolidinyl, piperidinyl, imidazolyl, isoxazolyl, pyrazolyl, dihydropyrazolyl, pyridyl, pyrrolyl, dihydropyrrolyl, phenyl, morpholinyl, thiazolyl, oxazolidinyl, imidazolidinyl, oxaazabicyclooctyl, and oxazepanyl, which may optionally be substituted by 1 to 4 substituents selected from a halogen atom, a hydroxy group, a cyano group, an oxo group, a C 1-10 alkoxy-carbonyl group, a C 1-10 alkoxy group which can be substituted by halogen, provided that a group represented by the formula where p is 1 to 4, which may be optionally substituted, is excluded; C 3-12 cycloalkyl, dihydropyranyl, tetrahydropyranyl, azetidinyl, pyrrolidinyl, piperidinyl, imidazolyl, isoxazolyl, pyrazolyl, dihydropyrazolyl, pyridyl, pyrrolyl, dihydropyrrolyl, phenyl, morpholinyl, thiazolyl, oxazolidinyl, imidazolidinyl, oxaazabicyclooctyl, and oxazepanyl, R 1 is a C 1-4 alkoxy group which can be substituted by one or more substituents selected from a C 1-4 alkoxy group, a halogen atom and a C 3-7 cycloalkyl group, wherein the optionally substituted C 1-4 alkoxy is radiolabeled with 11 C or 18 F; R 2 is a halogen atom, a C 1-10 alkyl group which can be substituted or a C 6-14 aryl group which can be substituted; R 3 is a halogen atom, a C 1-10 alkyl group which can be substituted or a C 1-10 alkoxy group which can be substituted; m is 0 or 1; and n is 0 to 3;