PURPOSE: To improve the working efficiency and achieve a high degree of machining of a flange by bringing a male die closely to a female die, cutting the outer periphery of a flange between a cut step and a receiving step and thereafter pressing the flange between a press part of the male die and that of the female die.

CONSTITUTION: A plastic sheet with a thickness required for formation of a vessel is heated in advance to form a main body 20 of the vessel having a flange 21 by means of a mold, after which the main body 20, in a reversed state, is fitted on a guide 19 provided on a male die 11, while the body 20 is fixed at a specified position by sucking it through a vacuum channel 18. Next, a female die 12 is brought closely to the male die 11 and, at first, the outer periphery of the flange 21 is cut between a cut step 32 of the die 11 and a receiving step 33 of the die 12 so that the width of the flange 21 is adjusted to have a length L. Then, the die 12 is further advanced toward the die 11 to press the flange 21 between a press part 30 of the die 11 and a press part 31 of the die 12, so that the flange 21 has a specified thickness.