A prismatic secondary battery includes an electrode assembly having a cathode, an anode, and a separator mounted in a prismatic battery case in a sealed state, the battery case including a hexahedral case body open at the top thereof, the hexahedral case body being made of a metal material, and a case cap coupled to the open top of the case body, the case cap having an electrode terminal, wherein the case body is provided at a region exhibiting a stress distribution equivalent to 40% or less of the maximum stress of the case in a high pressure state with a line-shaped safety groove configured to be ruptured when high pressure occurs in the battery, and the safety groove is configured such that a residual thickness of the case body is decreased toward the middle from opposite ends of the safety groove in the longitudinal direction.