To provide a highly flame-retardant polyphenylene ether-based resin composition not containing halogen or phosphorus atom.

This flame-retardant resin composition comprises (III) 0.1 to 50 pts.wt. of a polymer comprising silicon, boron and oxygen atoms, having a structural frame substantially formed of silicon-oxygen bond and boron-oxygen bond, and having aromatic groups in the molecule and 100 pts.wt. of a resin comprising (I) 30 to 100 pts.wt. of a polyphenylene ether resin and (II) 0 to 70 pts.wt. of an aromatic vinyl resin.