PURPOSE: To comparatively easily repair a membrane regardless of the kinds thereof, by heat-welding the membrane with a hot plate on condition that the upper half and the lower half of a repair device arranged in the upper and lower side of a membrane to be repaired are fixedly held by an electromagnet.

CONSTITUTION: An electromagnet 3 and a magnetic material 15 are attracted and fixed as shown by an arrow 18 by actuation of the electromagnet 3 to secure fixedly the upper half 1 of the repair device with the lower half thereof. In this condition, a piston-cylinder is actuated to lower the hot plate 8 in a direction indicated by the arrow 19 in order to interpose and press the membranes 16, 17 between hot plates 8, 12. And electric current is applied in the heating coil of the hot plates 8, 12 to heat-seal the membranes 16, 17 for 90-120sec. at about 370°C for instance. Thereafter, the membranes 16, 17 are cooled for 60-90sec. by the coolant flowing in a cooling water path of the hot plates 8, 15. This procedure is repeated at another place (other places than the superimposed part by the membranes 16, 17).