So far, the olefinic thermoplastic elastomers have been produced by a mechanical mixing method in which ethylene-propylene rubber (hereinafter referred to as EPR) or ethylene-propylene terpolymer (hereinafter referred to as EPDM) and thermoplastic resins such as polypropylene, are kneaded together through an extruder, and a method (polymerization method) in which the two components are prepared th