PURPOSE: To remove deflection and distortion by plural number of forming and form a flat heat transmitting plate by making repeated press forming to lap a continuous groove on the periphery of a heat transmitting plate and a part of undulatory part inside of it.

CONSTITUTION: In the first process of pressing, plural undulatory parts 5 consisting of crests and troughs are formed in the head leaving a space near the fluid passage hole in the upper part, and a linear continuous groove 6 of a specified length, which is a convex stripe when looked from the back and serves as a gasket insertion hole, and an undulatory parts 7 are formed on the peripheral part. At this time, in vertical direction of the heat transmitting plate 8, projecting part of the press die is formed to shallow ship bottom shape toward unworked part. In the second process, using the same die as used in the first process, pressing is made so as to lap the undulatory parts 5, 7 and a part of the continuous groove 6 formed in the first process. At this time, the part which is worked shallowly in the first process is deepened to the depth of undulatory part and groove.