PURPOSE: To prevent a welding dust particle from intruding into a container by a method wherein a protruding part corresponding to a groove part of a base is formed at an opening edge part of a cover and a resistance welding operation is executed.

CONSTITUTION: When a peripheral edge part of a base 11 is resistance-welded, a part in which a welding current flows is specified; a protruding bank 16 is formed in a ring shape so as to surely execute a sealing operation; a groove 19 is engraved in a ring shape at an inner periphery of the protruding bank 16; a protruding part 18 is formed in a ring shape at an opening edge of a cover 17 so as to correspond to the groove 19 of the base 11. In a state that the protruding part 18 of the cover 17 has been inserted into the groove 19 of the base 11, the peripheral edge part is sandwiched by welding electrodes; a welding current is applied; a resistance welding operation is executed; this assembly is sealed airtightly. Thereby, a welding dust particle produced when the welding current is applied is stopped by the protruding part 18 which has been inserted into the groove 19; it is possible to prevent the particle from intruding into a container.