The present invention relates to a polyester resin composition which has an excellent infrared light-transmitting property, and is excellent in heat resistance and low gas emission, and is suitable for use as black colored design parts (particularly as lamp parts). According to the present invention, there is provided a polyester resin composition containing, per 100 parts by mass of a polyester resin, 0.005 to 20 part(s) by mass of an inorganic filler that has an average particle size of 3 μm or less, 0.05 to 3 part(s) by mass of a polyfunctional glycidyl group-containing styrene polymer and 0.5 to 3 part(s) by mass of an infrared light-transmitting black dye, wherein the polyester resin composition satisfies the following requirements (1) and (2). (1) An average value of transmittances, for wavelengths of 800 to 1100 nm, of a flat plate that is obtained from the polyester resin composition and has a thickness of 2 mm is 5% or higher and less than 20%. (2) Color-L≤7.