Abstract: Organic phosphoric acid ester derivatives represented by the formula (I): ##STR1## wherein R and R', which may be the same or different, each represents an alkyl group, and A represents a group of atoms necessary to complete together with the two carbon atoms to which they are attached, a 6-membered heterocyclic group containing an oxygen atom as a heteroatom; a process for preparing the same and insecticidal, miticidal or nematocidal compositions containing the same as an active ingredient are disclosed; the compositions exhibit a high ability to control insect pests, mites, nematodes and unhygienic insects harmful to agricultural and horticultural crops.