A sealed prismatic battery according to the present invention comprises a laterally long cap sealing an opening of an upper surface of a battery can and a safety vent placed on the cap and having a thin-walled valve body which opens in an abnormal rise of a battery internal pressure. The safety vent has an oval-shaped circumferential groove formed inside an oval-shaped coining section provided on an outer surface of the cap and on one end side in a lateral direction of the cap and a thin-walled valve body surrounded thereby, and the oval-shaped circumferential groove is composed of lateral side sections parallel to the lateral direction of the cap and circular arc sections connecting these lateral side sections, with one lateral side section, other lateral side section, and the thin-walled valve body of the safety vent respectively having a thickness increased in this order.