By using a hydraulic forming device 20, a metal plate body 11 is placed on a support section 21b with a hollow section 21a formed on a lower die 21 filled with hydraulic fluid A. Subsequently, a blank holder 23 is lowered to clamp the peripheral edge section of the placed metal plate body 11 with the support section 21b. Then, the upper die 22 is lowered relative to the lower die 21 to pressedly deform the central section of the metal plate body 11 and to compress the hydraulic fluid A to increase the fluid pressure. A rib-like convex section 12 is transferred onto the metal plate body 11 by the increased fluid pressure of the hydraulic fluid A and a formed section 22a. Further, the fluid pressure of the hydraulic fluid A is kept for a predetermined time. Then, the increased fluid pressure of the hydraulic fluid A is released.