A flame-retardant resin composition comprises a base resin (A), an organic phosphorus compound (B) having a unit represented by the following formula (1a), and a flame-retardant auxiliary (C). Wherein Ar represents an aromatic hydrocarbon ring or a nitrogen-containing aromatic heterocycle; X1 represents an oxygen atom or a sulfur atom; Y1 and Y2 are the same or different and each represents a hydrocarbon group, an alkoxy group, an aryloxy group, or an aralkyloxy group; Z1 represents an alkylene group, or a nitrogen-containing bivalent group corresponding to an alkylamine; Y1 and Y2 may bind to each other, and Y1 and Y2 together with the adjacent phosphorus atom may form a ring; “a” denotes 0 or 1; and “b” denotes an integer of 1 to 6.