NOVELTY - The plate has a pair of mouths attaching positive and negative electrode terminals (15, 17), where one of mouths is formed on an end side along a longitudinal direction of the sealing plate, and the other mouth is formed on another end side. A gas release valve and an electrolyte pour hole are provided between the mouths, and the sealing plate including an identification code formed on a concaved flat face. The sealing plate includes a back face having a convex portion at a position corresponding to the concaved flat face.    USE - Sealing plate e.g. rectangular shape sealing plate, chamfered rectangular shape sealing plate, rounded rectangular shape sealing plate and oval shape sealing plate, for a prismatic secondary battery e.g. prismatic nonaqueous electrolyte secondary battery, used as a power supply for driving portable electronic equipment e.g. cell phone such as smartphone, portable computer, PDA and portable music player, and for a driving electric vehicle (EVs) and hybrid electric vehicle (HEV) and PHEV, and in a stationary storage battery system for suppressing variation in output power of photovoltaic generation and wind power generation, and for peak shifts in system power to store electric power during night time and to use electric power during daytime. Can also be used for a prismatic secondary battery e.g. nickel-hydrogen secondary battery, using an aqueous electrolyte.    ADVANTAGE - The plate with reduction in flexural strength with respect to the longitudinal direction of the sealing plate is small in a concaved portion with the prismatic secondary battery to suppress deformation of the sealing plate while internal pressure of the prismatic secondary battery is increased, and stabilize working pressure of the gas release valve. The prismatic secondary battery does not generate a level difference in the fitting portion between the sealing plate and the battery outer body, which enables uniform laser-welding of the fitting portion between the sealing plate and the battery outer body. The convex portion is formed on a back face when the concaved flat face is formed by forging, where the portion increases the flexural strength of the sealing plate along the longitudinal direction, and stabilizes working pressure of the gas release valve.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a prismatic secondary battery.    DESCRIPTION OF DRAWING(S) - The drawing shows a front view of a prismatic nonaqueous electrolyte secondary battery.    Positive electrode terminal (15)    Negative electrode terminal (17)    Insulating elements (18, 19)    Anti-rotation protrusions (38, 40)    Auxiliary protrusions (39, 41)