A novel herbicide having remarkable herbicidal activities against undesiredplantsis provided.A pyridazinone compound represented by the formula (l) or its salt:(see formula I)wherein X is -O-, -S-, -SO-, -SO2- or -N(Y)-； Q is monocyclic aryl which maybesubstituted by Z, monocyclic heteroaryl which may be substituted by Z or thelike； Y is ahydrogen atom or alkyl； Z is halogen, alkyl or the like； R1 is alkyl, alkenylor the like； R2is a hydrogen atom, alkyl or the like； R3 is halogen, hydroxy or the like； R4is a hydrogenatom, alkyl or the like； and n is an integer of from 0 to 4.
