A plate-shaped workpiece forming method of post-machining a pocket (3) on a curved inner surface of a plate-shaped workpiece (2) in a state where the plate-shaped workpiece (2) curved by a curving machine (10) is spread flat. The method includes a curving step (A) of setting a net curve radius (R0) obtained by adding a curve radius contraction amount (R1) due to spring-in to a finished curve radius (R) of a plate-shaped workpiece (2), taking into account an amount of contraction of the curve radius of the plate-shaped workpiece (2) between before and after machining of a pocket (3) due to spring-in, and curving the plate-shaped workpiece (2) so as to achieve the net curve radius (R0); and a pocket machining step of post-machining the pocket (3) by flatly spreading the curved plate-shaped workpiece (2).