Reactive flame retardant and flame-retardant resin processed article

A reactive flame retardant, comprising an organophosphorus compound represented by the following general formula (I) or (II), the organophosphorus compound having a terminal unsaturated group:

• where X1 to X3 each independently represent —OH or a group represented by the following formula (A), X4 represents a single bond or a group represented by the following formula (B), and one or more of X1 to X3 each represent a group represented by the following formula (A) and containing CH2═CH—CH2— at a terminal of the group;
• where R1 represents CH2═CH—CH2—, an aryl group having 12 or less carbon atoms, or an aralkyl group having 12 or less carbon atoms, R2 and R3 each independently represent a hydrogen atom, CH2═CH—CH2—, an aryl group having 12 or less carbon atoms, or an aralkyl group having 12 or less carbon atoms, and Y represents a single bond, —NH—, or —O—;
• where R4 to R9 each represent a group selected from phenyl, benzyl, α-naphthyl, β-naphthyl, p-biphenyl, phenoxy, benzyloxy, α-naphthoxy, β-naphthoxy, p-biphenyloxy, (CH2═CH—CH2)2N—, CH2═CH—CH2NH—, CH2═CH—CH2O—, and CH2═CH—CH2— groups, R10 represents H or CH2═CH—CH2—, at least one of R4 to R10 represents a group containing CH2═CH—CH2—, at a terminal of the group, and R4 to R9 may be identical to or different from one another.