A thermoplastic polyester resin composition, comprising: 0.1 to 10 parts by weight of a multivalent hydroxyl compound (B) having five or more hydroxyl groups and 0.1 to 10 parts by weight of a multifunctional epoxy compound (C), relative to 100 parts by weight of a thermoplastic polyester resin (A); wherein (B) is at least one species selected among sugar alcohols, cyclitols, monosaccharides, disaccharides and oligosaccharides; wherein (C) is a bisphenol-type epoxy compound and/or a novolac-type epoxy compound; wherein (A) is at least one species selected among polyethylene terephthalate, polypropylene terephthalate, polybutylene terephthalate, polypropylene naphthalate, polybutylene naphthalate, polypropylene isophthalate/ terephthalate, polybutylene isophthalate/ terephthalate, polypropylene terephthalate/ naphthalate and polybutylene terephthalate/ naphthalate; and wherein the thermoplastic polymer resin composition satisfies a following requirement (i): (i) a ratio of parts by weight of the multivalent hydroxyl compound (B) having five or more hydroxyl groups to parts by weight of the multifunctional epoxy compound (C) ((parts by weight of the multivalent hydroxyl compound (B) having five or more hydroxyl groups relative to 100 parts by weight of the thermoplastic polyester resin (A)) / (parts by weight of the multifunctional epoxy group (C) relative to 100 parts by weight of the thermoplastic polyester resin (A))) is 2 to 5. 0.1 to 10 parts by weight of a multivalent hydroxyl compound (B) having five or more hydroxyl groups and 0.1 to 10 parts by weight of a multifunctional epoxy compound (C), relative to 100 parts by weight of a thermoplastic polyester resin (A); wherein (B) is at least one species selected among sugar alcohols, cyclitols, monosaccharides, disaccharides and oligosaccharides; wherein (C) is a bisphenol-type epoxy compound and/or a novolac-type epoxy compound; wherein (A) is at least one species selected among polyethylene terephthalate, polypropylene terephthalate, polybutylene terephthalate, polypropylene naphthalate, polybutylene naphthalate, polypropylene isophthalate/ terephthalate, polybutylene isophthalate/ terephthalate, polypropylene terephthalate/ naphthalate and polybutylene terephthalate/ naphthalate; and wherein the thermoplastic polymer resin composition satisfies a following requirement (i): (i) a ratio of parts by weight of the multivalent hydroxyl compound (B) having five or more hydroxyl groups to parts by weight of the multifunctional epoxy compound (C) ((parts by weight of the multivalent hydroxyl compound (B) having five or more hydroxyl groups relative to 100 parts by weight of the thermoplastic polyester resin (A)) / (parts by weight of the multifunctional epoxy group (C) relative to 100 parts by weight of the thermoplastic polyester resin (A))) is 2 to 5. (i) a ratio of parts by weight of the multivalent hydroxyl compound (B) having five or more hydroxyl groups to parts by weight of the multifunctional epoxy compound (C) ((parts by weight of the multivalent hydroxyl compound (B) having five or more hydroxyl groups relative to 100 parts by weight of the thermoplastic polyester resin (A)) / (parts by weight of the multifunctional epoxy group (C) relative to 100 parts by weight of the thermoplastic polyester resin (A))) is 2 to 5. <EOS>