The present invention lies in a flame-retardant rubber composition comprising, as essential components, a synthetic rubber (a component A), a flame-retardant (a component B), sulfur (a component C) and a polyfunctional monomer la component D), wherein an amount of the component B is 10 to 200 parts by masse an amount of the component C is 0.1 to 15 parts by mass and an amount of the component D is 3 to 30 parts by mass, all relative to 100 parts by mass of the component A. It is a technical task of the present invention to provide a flame-retardant rubber composition which has excellent flame retardancy and can give a fully satisfactory mechanical strength (particularly, tensile strength at break) even when subjected to steam cure generally regarded to be difficult to impart a strength.