PURPOSE: To improve safety and reliability by pressure-holding an elastic unit for closing a battery case exhaust hole, and further securing holding plates having a through port and a battery case by providing a gas bleeding clearance therebetween so as to prevent bursting by serving as a reversible safety valve of good pressure response.

CONSTITUTION: A sheathing can 1 and metal-sealed covers 2 having a battery case exhaust hole are welded and built in with a generating element, and elastic units 3 of EPDM rubber are mounted from above on the exhaust hole of the cover 2 and pressed by holding plates 4 having an opening part in the center. Next by resistance- welding the covers 2 and protrusion parts 5 of the holding plates 4, a gas bleeding clearance is formed by the protrusion parts 5 between the covers 2 and the holding plates 4, to constitute a safety valve. This safety valve is operated as a reversible valve at low pressure time of about 3 to 15kg/cm2, and when a battery internal pressure rises, the elastic unit 3 is lifted to discharge gas from the gas bleeding clearance. When the battery internal pressure exceeds 15kg/cm2, the elastic unit 3 is discharged from the opening part of the holding plate 4, to efficiently discharge the gas. Thus by preventing bursting, safety, reliability and leak resistance are improved.