Abstract: A flame-retardant agent composition containing an organophosphorous compound represented by the general formula (I): wherein R1 and R2 each independently represent an alkyl group having 1 to 5 carbon atoms; R3 and R4 each independently represent a hydrogen atom or an alkyl group having 1 to 5 carbon atoms; Y represents a bond or a group —CH2—, —C(CH3)2—, —S—, —SO2—, —O—, —CO— or —N?N—; k represents 0 or 1; and m represents an integer of 0 to 4 and having a property such that a content of a compound represented by the formula (II): wherein R1, R2 and R3 are as defined in the general formula (I) is 1.8% by area or less when the organophosphorous compound is determined by gel permeation chromatography (GPC).