PURPOSE: To provide a heat transfer function to a satisfactory extent and miniaturize the size by making smaller the inside diameter of a vaporizer pipe substantially equivalent to a convection flow zone of a medium of the vaporizer pipe than that in a boiling zone of the medium.

CONSTITUTION: The inside diameter d2, which is about a half of a tube 9b equivalent to a medium convection flow zone of a vaporizer pipe is designed to be smaller than the inside diameter d1 which is about a half of a tube 9a substantially equivalent to a medium boiling zone 11. The tube 9a and the tube 9b are welded with each other, thereby constituting one vaporizer pipe. As a result, an area submerged to a medium liquid 4, or more particularly, the volume of an area substantially equivalent to the medium convection zone 10 is smaller than a prior art one and the medium liquid 4 to be filled up is acceptable even if it is small. A thermal capacity by a high temperature gas is increased more than a gas passage in the medium boiling zone 11, thereby speeding up the heating of the medium liquid 1. This construction makes it possible to provide a heat transfer function to a satisfactory extent and miniaturize the size.