A steel plate reinforcing sheet that can reduce distortions of the steel plate without reducing its reinforcing property and can also prevent abrupt deformations at end portions thereof with respect to a direction extending along a curving direction of a curved steel plate. The steel reinforcing sheet features a generally rectangular shape, as defined by a length and a width, and two end portions on either side of an imaginary line which bisects the length of the steel plate reinforcing sheet. The steel plate reinforcing sheet is a laminate including a restricting layer and a reinforcing layer, wherein each end portion of the restricting layer includes at least one slit. Each slit extends directly or indirectly across the entire width of the steel plate reinforcing sheet, and the two slits closest to the imaginary line define a minimum distance between slits. The minimum distance is parallel to the length of the steel plate reinforcing sheet and equivalent to 40% or more of the length of the steel plate reinforcing sheet.