To reduce stress and stress concentration in a process of forming a cavity in a metal plate, especially in a process of cutting a projecting part.

In this method, a recessed part 2 is formed from one surface 10a of the metal plate 10 by press forming by a press, and a projecting part 3 projected on the other surface side 10b of the metal plate 10 due to the formation of the recessed part 2 is cut by a cutting tool, thereby forming on the bottom surface a cavity having a bottom thinner than that of the metal plate 10. The projecting part 3 is cut from both sides by a pair of cutting tools 13 and 14 of which cutting directions are different and face to each other.