A flame retardant comprising a phosphazene compound, a polyphenylene oxide-series resin, and if necessary a styrenic resin and/or a nitrogen-containing compound imparts flame retardancy to a polyalkylene arylate-series resin. The phosphazene compound comprises at least a crosslinked phenoxyphosphazene compound. The obtained polyalkylene arylate-series resin composition shows high flame-retardancy.