To simplify the constitution of a heat exchanger and reduce the size and weight of the exchanger, and then, to protect the exchanger from generating dust and vibrating by closely fitting and fixing power unit attaching plates made of a material having high heat conductivity to the surfaces of heat radiating bodies on the opposite side of thermoelectric conversion elements and attaching the power units of the conversion elements to the plates.

The heat collected by thermoelectric conversion elements 2, 2,... is radiated to the outside by cooling water passed through heat radiating bodies 6 and 6. The cooling water simultaneously absorbs the heat generated from a power unit 20 composed of a DC-AC converter, substrate, etc., from the heat radiating bodies 6 and radiates the heat to the outside. Namely, the heat generated from the power unit can be radiated together with the heat collected by the elements 2. Therefore, the temperature of a liquid can be regulated quickly in response to the fluctuation of the temperature and the installed space of a heat exchanger can be reduced by simplifying the exchanger remarkably, because the temperature of the liquid can be regulated by directly cooling the liquid without providing any additional forcibly cooling device nor through a constant temperature oven.