PURPOSE: To permit to fill with a good operability by a method wherein the heat pipe is evacuated by the operation of a vacuum circuit and is filled with the operating fluid from an operating fluid filling device, thereafter, the opening thereof is sealed temporarily by forcing an elastic plug thereinto and is sealed perfectly by screwing the elastic plug.

CONSTITUTION: The operating fluid filling device 4 is closely attached to the opening 2 of the heat pipe 1, the vacuum pump 12 is driven and a valve 11 is opened to evacuate it, then, the valve 11 is closed when the heat pipe 1 and a cylinder 7 are evacuated, and, subsequently, the valve 14 is opened to fill the heat pipe with a specified volume of the operating fluid 19. Thereafter, the valve 14 is closed and a switching valve 16 is switched to force the U-cup-shaped soft polyethylene 9 into the opening 2 by the operation of a piston 8, thereby sealing it temporarily. Under this consition, the operating fluid filling device 4 is removed, an anaerobic sealing material is coated on the outer diametral male screw thread 3 of the opening 2 and a carbon steel box nut is screwed to seal it perfectly. Thus, the filling and the sealing of the heat pipe are effected with a good operability.