To form a container having no thermally bonded flap by simple constitution by arranging a thermal bonding contour line determining the boundary of a pouch portion to the inside of a cavity when strips of a film that form the pouch portion are inserted in a secondary mold, and eliminating the strip parts protruding from the side surface and bottom of the container.

Strips 3a, 3b are thermally bonded excepting the region in a cavity 11 in a thermal bonding station by closing mold halves 9a, 9b and a pouch portion 12 is formed to the thermally non-bonded part 11. Next, compressed air is injected into the pouch portion 12 by the nozzle of a blow molding head in the thermoforming station succeeding to a cooling station and a preheating station to mold a container 30. The thermally bonded part 17 determining the boundary of the pouch portion 12 is present in the molding cavity in this thermoforming stage. Therefore, a cavity 17a is formed to the thermally bonded part 17 during inflation to receive the thermally bonded part 17 to be substantially removed.