The present invention discloses a pyrrolopyridazinone compound represented by the formula (1):wherein R1 represents C1-C2
alkyl group or halogeno C1-C2
alkyl group,R2 repersents C3-C5 cycloalkyl group, (C3-C5 cycloalkyl)C1-C2
alkyl group or C1-C3 alkyl group,R3 represents
hydrogen atom, or
methylene group or cis-
vinylene group for forming substituted
oxygen-containing hetero ring in combination with group —O—R2,R4 represents
hydrogen atom,
halogen atom, C1-C8 alkyl group, C2-C6 alkenyl group, C2-C6 alkynyl group, hydroxy C3-C6 alkenyl group, hydroxy C3-C6 alkynyl group, C1-C6 alkyl group substituted by
substituent(s) selected from
Substituent group (a), C3-C6 cycloalkyl group which may be substituted by
substituent(s) selected from
Substituent group (b), “C1-C3 alkyl group which is substituted by C3-C6 cycloalkyl group which may be substituted by
substituent(s) selected from
Substituent group (b), and which may be substituted by a
hydroxy group”, an aromatic ring group or heteroaromatic ring group each of which may be substituted by a substituent(s) selected from Substituent group (c) or “C1-C2 alkyl group which is substituted by aromatic ring group or heteroaromatic ring group each of which may be substituted by group(s) selected from Substituent group (c), and which may be substituted by a
hydroxy group”,Substituent group (a) represents a
halogen atom,
hydroxy group, cyano group, carboxy group, C1-C5
alkoxy group, halogeno C1-C4
alkoxy group, C3-C6 cycloalkoxy group, (C3-C6 cycloalkyl)C1-C2
alkoxy group, C1-C4 alkoxycarbonyl group, C2-C4 alkanoyl group, C2-C4 alkanoyloxy group or C1-C4 alkyl-substituted amino group,Substituent group (b) represents a hydroxy group or a
halogen atom,Substituent group (c) represents a halogen atom, a hydroxy group, a cyano group, a nitro group, a carboxy group, C1-C5 alkoxy group, C1-C4 alkoxycarbonyl group, C2-C4 alkanoyloxy group, C1-C4 alkyl-substituted amino group or a C1-C4 alkyl group which may be substituted by a substituent(s) selected from the group consisting of (a halogen atom, a hydroxy group and a carboxy group),or a pharmaceutically acceptable salt thereof.