Secondary batteries comprise, for example, a battery case containing an electrolyte and a battery case lid closing an opening of the battery case.  The battery case lid mainly includes a substrate section welded to the battery case and an explosion-proof valve for releasing the internal pressure from inside the battery case when abnormality occurs to the electrolyte.  The explosion-proof valve is thinned compared to the substrate section and designed to rupture when the internal pressure exceeds a predetermined value. 
Conventionally, in a case where the explosion-proof valve is formed, the explosion-proof valve is joined to the substrate section by, for example, welding.  When the explosion-proof valve is joined to the substrate section by welding, however, it is difficult to make the thickness of the joining portion uniform.  This gives rise to a problem that an operational pressure at which the internal pressure is released cannot be made constant.  In addition, there is another problem that the welding involves complicated joining operation. 
Patent document 1 discloses a method of forming an explosion-proof valve by press-working a single metal plate.  In the method of Patent document 1, a single metal plate is coined to form a reduced thickness section thinner than a substrate section, and then, a groove is formed in the reduced thickness section along the periphery thereof.  A portion of the reduced thickness section where the groove is formed is even further reduced in thickness compared to the remainder of the reduced thickness section in order to ensure the rupture of the explosion-proof valve when the internal pressure exceeds a predetermined value.  In addition, in the method of Patent document 1, after the groove is formed, annealing is performed so that the operational pressure of the work-hardened explosion-proof valve can be controlled.