High-density polyethylene (HDPE) is known to be immiscible with isotactic polypropylene (PP) in the quiescent state. In an injection-molded blend of PP/HDPE (= 60/40 wt. ratio), a regularly phase separated structure with periodic distance of about 0.15 mu m was found. The regular structure could not be formed by a simple melt mixing of immiscible polymer pair but only by the spinodal decomposition (under zero shear rate in mold) from a single-phase mixture attained (by UCST depression or LCST elevation) in high sheer fields in an injection machine. The shear-dependent mixing and demixing mechanism was supported by transmission electron microscopy : the volume ratio of PP-rich region and HDPE-rich region being different from the charge ratio and the HDPE crystal lamellae developed in the PP-rich region. The mechanism was also supported by microscopic observation of the structure development (characteristic to the late stage of spinodal decomposition) during isothermal annealing above the melting point of PP; i.e. bicontinuous structure with nice regularity developed and grew up self-similarly.