A packaging machine for preparing sealed containers filled with contents each from a blank (11) having a thermoplastic synthetic resin layer over each surface thereof. The container is recessed in the central portion of its bottom so as to seat stably. A bottom forming end portion of the blank fitted around a mandrel (22) is folded flat and then pressed against the forward end surface of the mandrel by a press member with its press face to form the bottom of the container. The mandrel end surface defines a cavity for recessing the bottom central portion, and a projection complementary to the cavity is provided on the press face of the press member.
