To reduce cost and labor by providing such a method and device that projections are not formed at the right and left corners of a seal part while partly adopting the conventional heat-seal method when the opening end of a flexible tube is hermetically sealed to form a seal part.

This is a method hermetically sealing the opening end 8 of a flexible tube 2. The end 8 is linearly held by a temporarily clamping device 10 as a holding means and the linearly held end 8 is melted with heat by a heater as a melting means. And the ends are welded with pressure by a molding die 30 constituted of male and female dies 31, 32 having an arbitrary shape and mutually fitting the ends 8 melted with heat. When linearly holding the ends, the ends are positioned so as to correspond to the molding die 30 by a positioning means 15. This hermetically sealing method is used for the molding device of the ends of a flexible squeeze tube.