A hermetically sealed battery is provided, equipped with a opening sealing unit 10 including a Positive Temperature Coefficient (PTC) element without reducing the volume of the outside can, and capable of preventing the generation of a large current in case a short circuit occurs as well as enhancing operational security. The opening sealing unit 10 according to the invention comprises a bottom plate 11 for closing the opening of the outside can 18, a positive electrode cap 12 forming a space for storing a pressure valve, said positive electrode cap being used as a terminal for the positive electrode, a resilient valve 14 having a steel plate 14a on the upper surface, a spring 15 and a PTC element ring 16 disposed on the flange 12a in the positive electrode cap 12. An insulation gasket 17 is mounted onto the periphery of the opening sealing unit 10 to hermetically close the opening of the outside can 18.