PURPOSE: To raise durability and to suppress leakage of a heat-exchange fluid for along time by providing a connection flange bent to approach each other at two edge parts of adjoining plates, and welding a flat outside edge part of the connection flange for connecting.

CONSTITUTION: In order to connect adjacent plates 2, connecting flanges 5a and 6 are provided while two counter flat edge parts hit together, and outside edge parts are welded to form welding edge parts 9 and 10. A heat is transferred during welding of the connecting flanges 5 and 6 so that an enamel of an enamel layer of the plate 2 flows to an inside edge part of the connecting flanges 5 and 6, forming a closing enamel layer at a joint part between the connecting flanges 5 and 6 of both adjacent plates 2. Thus, with no post-process or enamel coating on the plate 2, a sufficient protective layer is formed around a flow-channel space. Suitable welding methods comprise a laser welding where heat supply is controlled while enamel flow is controlled well.