A resin composition characterized as containing (A) a synthetic resin having a melting temperature of 300.degree. C. or above and (B) a platy inorganic filler incorporated in the resin and having the following properties; pH of aqueous dispersion: 5.5-8.0, amount of extracted alkalis: Na 30 ppm or below and K 40 ppm or below, maximum diameter a: 50 .mu.m or below, thickness b: 1.0 .mu.m or below, and aspect ratio (a/b): 20 or above.