A novel 3(2H)-pyradazinone derivative of the formula (I) ##STR1## wherein, R represents an alkyl group having 1 to 4 carbon atoms substituted by a cycloalkyl group having 3 to 8 carbon atoms, an alkyl group having 1 to 4 carbon atoms substituted by a phenyl group which may be substituted or an alkyl group having 1 to 4 carbon atoms substituted by a heterocyclic group, R' represents a hydrogen atom, a halogen atom, an alkoxy group having 1 to 4 carbon atoms or a hydroxyl group having 1 to 4 carbon atoms, J represents any of various organic radicals. There is also provided a process for preparing said derivatives. These derivatives are useful as active ingredients of insecticidal, acaricidal and/or nematicidal compositions for agricultural and horticultural uses as well as of expellent compositions for pests parasitic on animals.