PURPOSE: To make weld traces at the inside of the cell as compared with the case where a laser beam irradiates from the surface side of the opposite side to the end face and further, to irradiate the pressure introducing pipe with the laser beam L even if it is bent.

CONSTITUTION: A flange part 1a formed on the end part of the pressure introducing pipe 1 is set on the supporting surface 4a of a holder 2 with the end face 1b of the pressure introducing pipe side directed below. The outside of the cell 5 is superposed on the surface 1d of the opposite side to the end face 1b of the flange part and further, the flange part 1a and the cell 5 are held by a clamper 6 and the holder 2. The clamper 6 is provided with a hole 6a having the larger diameter than a hole 5a provided on the cell, the periphery of the hole 5a of the cell exposed on the inside of this hole 6a is irradiated with the laser beam L and the cell 5 and the flange part 1a are subjected to laser beam welding from the inside of the cell 5.