PURPOSE: To attain the technical feasibility of avoiding the entry of hot water between an outer and an inner layer from an outside air inlet opening, stabilizing the inlet opening mechanically and obtaining a sanitarily used condition and a stable mechanical strength.

CONSTITUTION: At the time of blow molding into a bottle body 1 the parison consisting of a synthetic resin-made outer layer 2 forming the outer shell of a regular shape and an inner layer 3 made of a flexible synthetic resin and freely separably laid over the outer layer 2 to form the inner bag, a projecting piece is formed at the lower part of the bottle neck, the outer layer 2 corresponding to the forward end part of the projecting piece is cut off to expose the inner layer 3 so as to form an exposed part 6, thereby forming an opening part free from the deterioration of mechanical strength in a capped condition.