PURPOSE: To secure the sufficient melting time and molding time and improve the reliability of a final caulked section by using heat caulking punches of two different kinds of shape to ventilation holes formed by a plastic material and melt deforming the ventilation holes step by step.

CONSTITUTION: A first stage caulking punch 201 of trapezoid punch shape is lowered down toward a ventilation hole 102 formed by a plastic material and brought into contact with the ventilation hole 102, and the ventilation hole 102 is deformed while plastic is melted. The plastic is formed into the shape of a ventilation hole 103, and then a first stage caulking punch 201 is lifted up. Then, a second stage caulking punch 202 in the heated state is lowered toward a ventilation hole 103 and brought into contact with the ventilation hole 103, and the plastic is melted again to deform the ventilation hole 103. The plastic is formed into the shape of a sealing section 104 and then the second caulking punch 202 is lifted up. Then the heating to a sealing section 104 is discontinued and the plastic is solidified to complete the sealing.