A flame-retardant composition includes: a powdery acid-modified hydrogenated block copolymer (a) having a bulk density of 0.1 to 0.4 g/ml and which is a hydrogenated block copolymer modified by addition of an unsaturated carboxylic acid or a derivative thereof, the hydrogenated block copolymer comprises a polymer block A composed mainly of a vinyl aromatic compound unit and a polymer block B composed mainly of a conjugated diene unit, and has a weight average molecular weight of 50,000 to 500,000; a polyolefin resin (b); a non-aromatic rubber softener (c); and a metal hydrate (d). The flame-retardant composition contains the components (a), (b), (c) and (d) in proportions that satisfy the following relationships (1) to (3): 0.1≦Wa/(Wa+Wb)≦0.9  (1) 0.05≦Wc/(Wa+Wb+Wc)≦0.5  (2) 0.4≦Wd/(Wa+Wb+Wc+Wd)≦0.75  (3) wherein Wa, Wb, Wc and Wd represent masses of the acid-modified hydrogenated block copolymer (a), the polyolefin resin (b), the non-aromatic rubber softener (c) and the metal hydrate (d), respectively.