Heat exchanger and evaporator

A heat exchanger includes: a casing (21); an external cylinder (32) which is arranged inside the casing (21) with a clearance formed between the external cylinder (32) and an inner surface of the casing (21) and is supported in a cantilevered manner, has a closed end portion (32b), and forms a first fluid passage (81) between the external cylinder (32) and the casing (21); and an internal cylinder (40) which is arranged inside the external cylinder (32) with a clearance formed between the internal cylinder (40) and an inner surface of the external cylinder (32) and is supported in a cantilevered manner, is open at both ends, and forms a second fluid passage (71), which is turned around at the closed end portion (32b) of the external cylinder (32), between the internal cylinder (40) and the external cylinder (32).