PURPOSE: To obtain sufficiently durable strength to internal pressure and to prevent volume expanding materials due to the internal pressure or deformation from leaking out by coupling a main body with the joint part of a case by a high-energy density beam.

CONSTITUTION: The main body 11 has a through hole 12 and an upper chamber 13 formed at the center part, a lower chamber 15 is formed in the case 14, and volume expanding materials 17 and 18 which expand and contract with ambient temperature are charged in the upper and lower chambers 13 and 14 across a diaphragm 16. The diaphragm 16a has its outer peripheral part clamped between the main body 11 and case 14 and a projection part 16a formed at its outer peripheral part is fitted in the recessed part 11a of the main body 11. The main body 11 and case 14 are joined together projecting outward from the outer peripheral end of the diaphragm 16 and the outer peripheral part of the joint part A is coupled by soldering 19 by a laser. Therefore, neither the main body 11 nor the case 14 deforms owing to a high load at the time of the joining process and even if the internal pressure becomes extremely high by the expanding material 7, no gap is formed between the joined surfaces to prevent the expanding materials from leaking out.