A heat exchanger includes: a casing; an external cylinder which is arranged inside the casing with a clearance formed between the external cylinder and an inner surface of the casing and is supported in a cantilevered manner, has a closed end portion, and forms a first fluid passage between the external cylinder and the casing; and an internal cylinder which is arranged inside the external cylinder with a clearance formed between the internal cylinder and an inner surface of the external cylinder and is supported in a cantilevered manner, is open at both ends, and forms a second fluid passage, which is turned around at the closed end portion of the external cylinder, between the internal cylinder and the external cylinder.