[en] We developed a thermoplastic elastomer by blending symmetric polystyrene-block-polyisoprene-block-polystyrene (SIS) and asymmetric SIS block copolymer. This polymer blend is promising as an elastic film because it has superior physical properties, such as high modulus and recoverability, compared to symmetrical SIS homopolymers which have been used as adhesives. In order to elucidate the reasons for such excellent physical properties, its structure was investigated by time-resolved small angle X-ray scattering, transmission electron microscope observation, and SCF simulation in stretching process. In the film made by pressing close to industrial processing, the characteristic structural change of the styrene domain was observed with time-resolved small angle X-ray scattering. Furthermore, it was also found that the processing conditions have a strong influence on the phase separation structure. (author)