Disclosed herein is a plate-shaped secondary battery constructed in a structure in which an electrode assembly of a cathode/separator/anode structure is mounted in a battery case, and the battery case is sealed by thermal welding, wherein the battery case is provided at a sealing portion around an electrode assembly receiving part thereof with an exhaust guide sealing portion configured in a structure in which a width of the exhaust guide sealing portion decreases from an inside of the sealing portion toward an outside of the sealing portion such that a sealing force of the exhaust guide sealing portion is first released, when high-pressure gas is generated in a battery cell, and therefore, the high-pressure gas is exhausted outside, the exhaust guide sealing portion being thermally welded with a sealing force less than that of a sealing portion located at a remaining region (‘a remaining sealing portion’), and the battery case is further provided at a middle of the exhaust guide sealing portion with a bridge sealing portion extending in parallel to the electrode assembly for interconnecting opposite sides of the remaining sealing portion with a predetermined width, the bridge sealing portion being thermally welded with a sealing force greater than that of the exhaust guide sealing portion.