PURPOSE: To reduce cost by using a mold clamping device low in clamping force and simplifying the structure of a mold.

CONSTITUTION: A space 16 is formed between first and second molds and a resin 13 is injected into the space 16 from an injection nozzle 17 to form a resin lump. In this case, only the resin lump may be formed in the space 16 and it is unnecessary to mold a parison. Therefore, the clamping force for clamping the first and second molds may be low. Next, the first mold is replaced with a third mold and the second mold is moved to press the resin lump to mold the parison. Continuously, the second mold is replaced with a fourth mold having the molding surface corresponding to a molded product. Blow air is blown off from the third mold to expand the parison and a molded product having the shape along the molding surface of the fourth mold is produced.