The present invention relates to an olefinic thermoplastic elastomer composition which comprises a crystalline polyolefin resin (A), an ethylene-α-olefin-non-conjugated polyene copolymer rubber(B) and at least one rubber(C) selected from polyisobutylene, butyl rubber and a propylene-ethylene copolymer rubber having a propylene content of 50 mol% or more or an ethylene-α-olefin copolymer(D) having a density of 0.870 to 0.940 (g/cm 3 ) and an ethylene content of 85 mol% or more, said ethylene-α-olefin-non-conjugated polyene copolymer rubber(B) being a crosslinked thermoplastic elastomer composition, being obtained by random copolymerization of ethylene, an α-olefin having 3 to 20 carbon atoms and a non-conjugated polyene in the presence of a metallocene catalyst, (1) containing the unit (a) derived from the ethylene and the unit (b) derived from the α-olefin in a molar ratio of 40/60 to 95/5 [(a)/(b)], (2) having an iodine value of 1 to 50 and (3) having an intrinsic viscosity[η] measured in decalin at 135°C of 1.0 to 10 dl/g; and relates to a method for manufacturing the same.