Disclosed is a reactive flame retardant which provides a resin with excellent flame retardance even when it is added in a small amount while being prevented from bleedout. Also disclosed is a flame-retardant resin processed article obtained by using such a reactive flame retardant. An organophosphorus compound represented by the general formula (I) below, wherein at least one or more of X.sup.1-X.sup.3 represent a group containing phosphorus and having a terminal unsaturated group, is used as a reactive flame retardant which is reactive with resins. A flame-retardant resin processed article can be obtained by solidifying the resin composition containing the organophosphorus compound and then reacting it through heating or application of radiation.