A polypropylene-based resin composition of drastically improved dimensional stability realized by incorporating a polypropylene resin of specific structure with an inorganic filler of specific particle size, carbon fibers of specific fiber diameter and length, and, as required, an ethylene- and/or styrene-based elastomer. Another polypropylene-based resin composition of well-balanced properties of rigidity, heat-resistance, impact resistance and lightness even essentially in the absence of inorganic filler, realized by compositing a propylene/ethylene block copolymer of specific structure and well-balanced mechanical properties with an ethylene- and/or styrene-based elastomer, and carbon fibers of specific structure.