Abstract: There has been developed a safety valve installed on a small-sized cell vessel which remains hermetically sealed during the normal operation but which instantly releases the pressure in the inside of the vessel if, due to abnormalities, such as overcharging, the pressure is increased and reaches a prescribed value to prevent explosion or deformation of the cell vessel to evade the risk to the human being or destruction of the equipment. The metal foil type safety valve by the cold pressure welding of the present invention cold-pressure-welds the metal foil for sealing the air vent hole in the metal vessel. If the internal pressure in the vessel reaches the prescribed value, the metal foil is cleft to form an opening in the vessel to release the internal pressure.