PURPOSE: To obtain a heat exchanger, having a new structure capable of coping with the difference of thermal expansions between a heat transfer tube and a circular cylinder without employing any expansion joint, U-shaped heat transfer tube or a loose head, which have some troubles, and easy in the cleaning of the heat transfer tubes and the circular cylinder.

CONSTITUTION: One end of a heat transfer tube 5 is communicated with a high-temperature fluid chamber 1 and the other end of the same is provided with a heat transfer tube header 9 having a guide 10 while one end 6 of the circular cylinder 13, receiving the heat transfer tubes 5, is provided in the high- temperature fluid chamber 1. The other ends 8 of the heat transfer tubes are connected movably and air-tightly to the guide 10 through O-rings.