PURPOSE: To realize the lightweight water heater and at the same time facilitate its mounting by a structure wherein the water heater is formed by joining the outer periphery of two sheet plate-like members, which are deformable by gravity.

CONSTITUTION: The two deformable sheet plate-like members are jointed with each other at its outer periphery and at a part of its inner periphery so that the selling of the whole sheet plate is suppressed by denting locally at the jointed parts and the heights of resultant convex parts are made to be same. Furthermore, a part of the inner periphery is joined to each other so as to arrange itself to form a passage of water or a pipe is arranged so as to obtain a desired area.