PURPOSE: To make it possible to expand a floating bag for a short period of time by breaking through the sealing plate of a gas cylinder while pushing out a cutter by making use of a gas producer operated with a seawater battery.

CONSTITUTION: A sealing plate 22 is provided between a gas cylinder 10 and an inlet 24 on which a floating bag is mounted, which is broken through by means of a cutter 26. A seawater battery 50 generates electromotive force by being immersed in seawater, causing a gas producer 48 to be burned by a sparking plug 55. Hereby, a piston 45 is pushed out by gas pressure produced to permit a shear pin 46 to be sheared, allowing a cutter 26 to be operated. Since the seawater battery and the gas producer are used, operation time can be reduced compared with one obtained by using Japanese paper.