To provide a compact heat exchanger with high thermal efficiency.

A heat exchanger comprises a casing 1 having both ends closed, opposed flow chambers 3a and 3b assembled from both the ends of the casing 1, and provided at the center or a part near the center of the casing 1, a supply hole 5 formed in the flow chamber 3a, an exhaust hole 6 formed in the flow chamber 3b, a plurality of radiating pipes 7 provided on the periphery of the flow chamber 3b and passing through the casing 1, an upstream header part 8 to which an end of each radiating pipe is connected and a downstream header part 9 to which the other end of each radiating pipe is connected. Then, the water of a bathtub 19 is supplied from the flow chamber 3a side and the high temperature fluid of a boiler 17 is supplied from the upstream header 8 side.