Disclosed herein is a plate-shaped secondary battery configured to have a structure in which an electrode assembly of a cathode/separator/anode structure is mounted in a battery case in a sealed state by thermal welding, wherein a protuberance is formed at an outer edge sealed portion of an electrode assembly receiving part of the battery case such that the protuberance protrudes upward and downward from the sealed portion, and the protuberance extends along the outer edge of the electrode assembly receiving part continuously in a curved line or in a straight line in a state in which the protuberance is adjacent to the electrode assembly receiving part.