The component (b) in the olefin-based copolymer composition is a propylene-ethylene copolymer, and as monomer components constituting the copolymer, about 5 to 85 mol % of a monomer unit derived from ethylene and about 95 to 15 mol % of a monomer unit derived from propylene are preferable, and an ??-olefin other than ethylene and propylene, for example, 1-butene, 4-methyl-1-pentene, 1-hexene, 3-me