A thermoplastic polyester resin composition comprising 100 parts by weight of a thermoplastic polyester resin (A), 5 to 40 parts by weight of a phosphinate (B), and an organosiloxane (X); wherein the phosphinate is a calcium salt or aluminum salt of phosphine acid having the anion part having a particular structure; which, as the organosiloxane (X), comprises 1.5 to 10 parts by weight of an organosiloxane compound (X-1) or 0.01 to 5 parts by weight of an organosiloxane polymer (X-2), relative to 100 parts by weight of the thermoplastic polyester resin (A); and wherein the organosiloxane compound (X-1) comprises organic groups bonding to a silicon atom directly or through an oxygen atom, and 40 mole % or more of the organic groups are an aryl group, and the organosiloxane polymer (X-2) is in the form of solid at 25° C.