A pyrrolopyridazinone compound represented by the formula (1): wherein R2 represents a C1-C2 alkyl group or a halo-C1-C2 alkyl group, R2 represents a C3-C5 cycloalkyl group, a group (C3-C5 cycloalkyl) - C1-C2 alkyl or a C1-C3 alkyl group, R3 represents a hydrogen atom, or a methylene group or a cis-vinyl group, each of which is a group for the constitution of a heterocyclic ring containing oxygen substituted together with a group -O-R2, R4 represents a hydrogen atom, a halogen atom, a C1-C8 alkyl group, a C2-C6 alkenyl group, a C2-C6 alkynyl group, a C3-C6 hydroxy-alkenyl group, a group C3-C6 hydroxy-alkynyl, a C1-C6 alkyl group substituted with a substituent or substituents selected from the Group of substituents (a), a C1-C6 cycloalkyl group, which may be substituted with a substituent or substituents selected from the Group of Substituents ( b), "a C1-C3 alkyl group that is substituted with u n C3-C6 cycloalkyl group, which may be substituted with a substituent or substituents selected from the Group of Substituents (b), and which may be substituted with a hydroxy group ", an aromatic ring group or a heteroaromatic ring group, each of which may be one of those substituted with a substituent or substituents selected from the Group of Substituents (c), or "a C1-C2 alkyl group that is substituted with an aromatic ring group or a heteroaromatic ring group, each of which being substituted with a substituent or substituents selected from the Group of Substituents (c), and which may be substituted with a hydroxy group ", The Group of substituents (a) represents a halogen atom, a hydroxy group, a cyano group, a carboxy group, an alkoxy group C1-C5, a halo-C1-C4 alkoxy group, a C3-C6 cycloalkoxy group, a C1-C2 (cycloalkyl) group C1-C2 alkoxy, a C1-C4 alkoxycarbonyl group, a C2-C4 alkanoyl group, a group C2-C4 canoyloxy or an amino group substituted with C1-C4alkyl, the Substituent Group (b) represents a hydroxy group or a halogen atom, the Substituent Group (c) represents
