PURPOSE: To suppress a thermal effect such as the occurrence of a warp to the minimum and to reduce the quantity of heat required for welding by tapering away an end of a cover so that the periphery which is made to a weld zone becomes thinnier.

CONSTITUTION: Melting of the cover 2 is reduced and sufficient heat is also transmitted to the vessel 1, and thermal welding can be performed with the minimum quantity of heat by tapering away the end of the cover. When an angle of a taper becomes larger, since a molten region of the cover is extended, it is necessary to increase the output and the heat effect is easily produced to that extent. When the taper has the angle of ≥60°, the warp quantity increases and when it has the angle of ≤60°, the warp does not occurs. In addition, from the standpoint of strength, the larger angle is desirable but when the angle is 60°, the strength is reduced by only about ten percent in the case of 90° and even in the case of 45, the strength more than seventy percent in the case of 90° is held. Synthetically, the range of about 45 to 60° is desirable as an optimum value of this angle.