Abstract: Provided are: a pyridazinone derivative and/or a pharmaceutically acceptable salt thereof, which is useful as a therapeutic agent and/or a prophylactic agent for diseases in which Nav1.1 is involved and various central nervous system diseases; and a medicine containing the pyridazinone derivative and/or the pharmaceutically acceptable salt thereof as an active ingredient. A compound represented by formula (1) or a pharmaceutically acceptable salt thereof. [In the formula, M1 represents a saturated or partially unsaturated C4-12 carbocyclic group or the like; R1 and R2 independently represent a hydrogen atom or the like; M2 represents a group represented by formula (2a) or the like; X1a, X1b and X1c independently represent N or the like; X2, X3 and X4 independently represent CR3 or the like; A1 and A2 independently represent N or the like; and R3 represents a hydrogen atom or the like.