This invention relates to pyridazine derivatives represented by the formula (1): wherein R1 represents a (substituted) aryl group, R2 represents a phenyl group substituted at 4-position by a lower alkoxyl group or a lower alkylthio group, R3 represents a lower alkoxyl group, a halogenated lower alkyl group, a lower cycloalkyl group, a (substituted) aryl group, a (substituted) aryloxy group, a (substituted) nitrogen-containing heterocyclic ring residue, a (substituted) aminocarbonyl group or a lower alkylcarbonyl group, A represents a single bond, a lower alkylene group or a lower alkenylene group, X represents O or S, and the dashed line indicates that the carbon-carbon bond between the 4-position and the 5-position is a single bond or a double bond, or salts thereof; and also to medicines containing them as effective ingredients. These compounds have excellent inhibitory activity against interleukin-1β production, and are useful as preventives and therapeutics for immune system diseases, inflammatory diseases, ischemic diseases and the like.