NOVELTY - The plate has a pair of mouths at respective ends for attaching negative and positive electrode terminals (17,15). A gas release valve and an electrolyte pour hole are provided between the pair of mouths. A coining area is provided on front face around the pair of mouths to position insulating elements (18,19). Front face is provided with a groove from a position between the corresponding coining area and a long side edge to a position near the gas release valve along the plate long side edge.    USE - Sealing plate for prismatic secondary battery (claimed) e.g. prismatic nonaqueous electrolyte secondary battery used for electric vehicle (EV), hybrid electric vehicle (HEV) and stationary storage battery system. Can also be used in prismatic secondary battery using aqueous electrolyte, such as nickel-hydrogen secondary battery.    ADVANTAGE - The flow of metal during the formation of the pair of mouths for attaching a positive electrode terminal and a negative electrode terminal near respective ends of the sealing plate and the coining area on the front face near the mouths can be suppressed by the concomitant formation of groove between the coining area and the long side edge of the sealing plate, so that a sink mark or a shear drop is not generated in the coining area. Thus positioning performance of the insulating element when the insulating element is attached to the coining area can be improved effectively. The groove is formed with a large depth near the coining area and a small depth near the exhaust valve, thus the sealing plate can be easily produced. The reduction in flexural strength with respect to the longitudinal direction of the sealing plate is small in the concaved flat face, so that deformation of the sealing plate when the internal pressure of the prismatic secondary battery is increased can be completely suppressed and working pressure of the gas release valve can be stabilized.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) a method for producing a sealing plate for a prismatic secondary battery; and    (2) a prismatic secondary battery.    DESCRIPTION OF DRAWING(S) - The drawing shows a front view of the prismatic nonaqueous electrolyte secondary battery.    Prismatic nonaqueous electrolyte secondary battery (10)    Positive electrode terminal (15)    Negative electrode terminal (17)    Insulating elements (18,19)    Auxiliary protrusion (39)