Patented is a polyester resin composition that comprises, based on 100 weight % of the total polyester resin composition, 30 to 80 weight % of a polybutylene terephthalate resin. It also includes 10 to 40 weight % of another polyethylene terephthalate resin and 0.1 to 40 weight % of a calcined kaolin having an average particle diameter of 1.5 micro meter or less, 0.1 to 40 weight % of a spherical inorganic filler. A releasing agent comprises a montanic acid complex acid ester.