A 4-pyrazolyl phosphorous acid ester of the following formula, ##STR1## (R is a lower alkyl group; Y is a phenyl group which may optionally be substituted by a halogen, halogens, a lower alkyl group or a halogen and a lower alkyl group.) which is produced by reacting corresponding 4-hydroxylpyrazole compound with a trialkyl phosphite. The ester gives 4-pyrazolyl phosphorothioate by the reaction with an alkylsulfenyl halide.