The polyolefin-based resin composition (I) of the present invention comprises (A) 99.9 to 80% by mass of a propylene (co)polymer having (a) a melt flow rate (MFR) of 0.1 to 200 g/10 min, (b) a meso-pentad fraction [mmmm] of an ordinary temperature (25° C.) xylene-insoluble component of 97.0% or higher, and (c) a content of an ordinary temperature (25° C.) xylene-soluble component of 4 to 50% by mass; and (B) 0.1 to 20% by mass of a copolymer containing propylene chains of an isotactic structure, and copolymer chains constituted from at least two monomer units selected from the group consisting of ethylene, C3 to C20 α-olefins, cyclic olefins and aromatic vinyl monomers which are graft-bonded and/or block-bonded to said copolymer, and exhibits not only a high rigidity but also a highly enhanced impact strength and is well-balanced between the physical properties thereof.