PURPOSE: To precisely form a thin sheet metal material to a complicated three- dimensional shape by inserting and holding a thin plate stock by thick plate materials and bringing it to press forming in a die, and thereafter, heating and holding it as it is at a prescribed temperature.

CONSTITUTION: A thin plate stock 12 is inserted and held from both faces by a pair of thick plate materials 13, 14, and formed as one body by girth weld. At the time of press bending, a male die 15 and a female die 16 constituted of a curved surface which is shifted by a plate thickness portion of the plate materials 13, 14 from a curved surface to which the plate stock 12 is to be bent originally is used. The plate stock 12 and the plate materials 13, 14 which are heated and formed as one body are placed on the female die 16, and allowed to go along the dies 15, 16 by pushing in the male die 15 by a press. Subsequently, the plate stock 12 and the plate materials 13, 14 are carried into a heating furnace in a state that they are inserted and held by the dies 15, 16, heated up to a prescribed temperature and held for a prescribed time. In such a way, the plate stock 12 can be formed to a shape as per the die.