A thermoplastic polyester resin composition comprising 100 parts by weight of a thermoplastic polyester resin, and from 0.1 to 10 parts by weight of a novolac type epoxy resin represented by the following general formula; wherein in the general formula, X represents a divalent group represented by the general formula or shown above; wherein in the general formulae and, R 1 to R 4 each independently represents an alkyl group having from 1 to 8 carbon atoms or an aryl group having from 6 to 10 carbon atoms, and they may be the same or different from each other; and R 5 represents a hydrogen atom, an alkyl group having from 1 to 8 carbon atoms, or an aryl group having from 6 to 10 carbon atoms; wherein in the general formula, n represents a value greater than 0 and not more than 10; and wherein in the general formulae and, a, c, and d each independently represents a value of from 0 to 4, and b represents a value of from 0 to 3; wherein the thermoplastic polyester resin is a polymer or a copolymer comprising 50% by mole or more of a residue of an aromatic dicarboxylic acid or an ester-forming derivative thereof and a residue of an aliphatic diol or an ester-forming derivative thereof; and wherein the amount of the carboxyl end groups in the thermoplastic polyester resin, which is determined by the method given in the present specification, is 50 eq/t or less; and wherein the ratio (the blend concentration of epoxy groups/ the blend concentration of carboxyl end groups) of the blend concentration (eq/g) of the epoxy groups derived from the novolac type epoxy resin represented by the general formula to the blend concentration (eq/g) of the carboxyl end groups derived from the thermoplastic polyester resin, in the thermoplastic polyester resin composition, is from 1 to 8. wherein in the general formula, X represents a divalent group represented by the general formula or shown above; wherein in the general formulae and, R 1 to R 4 each independently represents an alkyl group having from 1 to 8 carbon atoms or an aryl group having from 6 to 10 carbon atoms, and they may be the same or different from each other; and R 5 represents a hydrogen atom, an alkyl group having from 1 to 8 carbon atoms, or an aryl group having from 6 to 10 carbon atoms; wherein in the general formula, n represents a value greater than 0 and not more than 10; and wherein in the general formulae and, a, c, and d each independently represents a value of from 0 to 4, and b represents a value of from 0 to 3; wherein the thermoplastic polyester resin is a polymer or a copolymer comprising 50% by mole or more of a residue of an aromatic dicarboxylic acid or an ester-forming derivative thereof and a residue of an aliphatic diol or an ester-forming derivative thereof; and wherein the amount of the carboxyl end groups in the thermoplastic polyester resin, which is determined by the method given in the present specification, is 50 eq/t or less; and wherein the ratio (the blend concentration of epoxy groups/ the blend concentration of carboxyl end groups) of the blend concentration (eq/g) of the epoxy groups derived from the novolac type epoxy resin represented by the general formula to the blend concentration (eq/g) of the carboxyl end groups derived from the thermoplastic polyester resin, in the thermoplastic polyester resin composition, is from 1 to 8.