There is provided a sealed prismatic battery having an opening vent excellent in reliability with use of a deformed state of a battery casing on occasion of abnormal increase in internal pressure of the battery.The battery casing is shaped like an oval quadrangle having a width in a direction of thickness smaller than a height in a longitudinal direction and a length in a lateral direction, and side walls of the casing that face in the lateral direction swell outward in the lateral direction so as to form circular-arc shapes in general. The opening vent is composed of a groove formed as a recess on the side wall. A distances from the upper end of the battery casing to the groove is set between 3 and 20% of the longitudinal height of the battery casing.