To obtain a resin composition having excellent heat resistance, weatherability and extrudability, and useful as a sizing material fitting on an outer wall of a house by containing a specific ratio of a polycarbonate resin and a polymethyl methacrylate resin.

This resin composition is composed of (A) 90-10 pts.wt. of a polycarbonate resin [e.g. the one obtained by a transesterification of a compound of the formula {R is H, a halogen or a 1-8C alkyl; X is a single bond, a 1-8C alkylene, S, 0, etc.; (n) and (n') are each 0-4} with a diester carbonatel and (B) 10-90 pts.wt. of a polymethyl methacrylate resin, and preferably further contains (C) an impact resistance improving material such as an ABS resin, an AES resin, an AAS resin, an ACS resin, an MBS resin or acrylic rubber in an amount of 1-50 pts.wt. based on 100 pts.wt. of components (A+B).