A polyalkylene terephthalate-series resin (e.g., polyethylene terephthalate-series resin, polybutylene terephthalate-series resin) is improved in flame retardancy by using a flame retardant containing a phosphazene compound and a phenolic resin. The phosphazene compound is a compound shown by the following formulae (1) and/ or (2), and/or a compound being the above compound crosslinked with a phenylene group or a crosslinking group shown by the following formula (3a). The proportion of the phosphazene compound to the phenolic resin is the former/the latter=5/95 to 95/5 (weight ratio). By means of the above flame retardant, a polyalkylene terephthalate-series resin can be highly improved in flame retardancy without deteriorating the properties. Further, by adding the other flame retardant (e.g., a nitrogen-containing flame retardant, a phosphate-series flame retardant), and a carbonizable resin, the flame retardancy can be enhanced. Mechanical Property In accordance with ASTM D-638 and ASTM D-790, the mechanical strength was determined with use of a test piece obtained by injection molding.