A thermoplastic elastomer composition dynamically vulcanized to a gelation rate of 50 to 95% which is superior in heat resistance and durability while maintaining flexibility and superior in air permeation preventive property which can be efficiently used as, for example, a pneumatic tire as an air permeation preventive layer. In particular, a thermoplastic elastomer composition having a reduced particle size of the domain rubber is produced by a material having a high rubber ratio by mixing a composition (C) mixed under conditions of a ratio of melt viscosities of the rubber composition (A)/resin (B) of 0.8 to 1.2 and the formula (φA/φB)×(ηB/ηA)