A novel herbicide having remarkable herbicidal activities against undesired plants is provided.A pyridazinone compound represented by the formula (I) or its salt:wherein X is —；O—；, —；S—；, —；SO—；, —；SO2—； or —；N(Y)—；； Q is monocyclic aryl which may be substituted by Z, monocyclic heteroaryl which may be substituted by Z or the like； Y is a hydrogen atom or alkyl； Z is halogen, alkyl or the like； R1 is alkyl, alkenyl or the like； R2 is a hydrogen atom, alkyl or the like； R3 is halogen, hydroxy or the like； R4 is a hydrogen atom, alkyl or the like； and n is an integer of from 0 to 4.
