A safety valve for a battery which is mounted on a sealing plate for sealing an opening portion of a gastight battery case, and which can rapidly release a gas in the battery when a battery internal pressure is elevated is provided. In a safety valve for a gastight battery, a thin-wall valve element is formed on a sealing plate which seals an opening portion of a battery case, and the valve element is opened when a battery internal pressure becomes a predetermined value or more so as to release a gas in the battery to the outside of the battery. The safety valve includes: an outer annular groove formed of an annular thin wall which is formed on the periphery of the valve element and is provided for releasing the whole valve element; an inner annular groove formed of an annular thin wall which is arranged in an area inside the outer annular groove; and a connecting groove formed of a straight thin wall which connects the outer annular groove and the inner annular groove to each other. A wall thickness of the inner annular groove and a wall thickness of the connecting groove are set larger than a wall thickness of the outer annular groove.