PURPOSE: To obtain a bottle whose thickness is uniform by obtaining a radiant intensity distribution suitable for heating parison, by providing a large diameter part on an end part of a cylindrical heater which is inserted within the parison, in a method molding a bottle through biaxial orientation blow by heating the parison at a suitable temperature for orientation.

CONSTITUTION: A large diameter part of a heater 2 is formed at positions corresponding to parison parts 11, 13 which become the bottom and shoulder parts of a bottle at the time of blow molding. Consequently, a distance L2 from a heater surface is large at a parison part 12 which becomes a belly part at the time of the blow molding, a distance L1 becomes small at the parison parts 11, 13 and the parison parts 11, 13 which are hard to be stretched are heated at the same temperature as that of the parison part 12 or more than that, which causes a satisfactory result. Although a part of the heater may be manufactured at a large diameter beforehand to form the large diameter part on the heater 2, it is simple and flexible to depend upon a method fitting attachments 21, 23 made of metal to an infrared cartridge heater.