PURPOSE: To accurately set the dimension of the mouth edge part of a container after thermal molding and to prevent the thermal deformation of the container at the time of heating, in molding the container, by making the temp. of the place corresponding to the mouth edge part of the container obtained in a male mold lower than that of the place corresponding to the side wall part of the container.

CONSTITUTION: The surface 101 (the place corresponding to the side wall part of an obtained container) of the body part 11 of a male mold 1 is heated to predetermined molding temp. by a cartridge heater 8 and a heat insulating layer 13 is provided between the mouth part 12 of the feamal mold 1 and the body part 11 thereof and the surface 102 (the place corresponding to the mouth edge part of the container) of the mouth part 12 is held to temp. lower than that of the surface 101. 1 As a result, the cooling degree of the mouth edge part of the container becomes large and the caliber of the container after molding becomes hard to expand. Thermal deformation is prevented in cosiderable quantity at the time of heating and cooking. In this case, high and low temps. are relative. The heat insulating layer 13 is composed of paper or porous ceramics.