PURPOSE: To make it possible for a heat exchanger to be highly capable, small size and yet capable of being installed at any place by utilizing liquid evaporating latent heat.

CONSTITUTION: A heat exchanger consists of a dual pipe, composed of an inner pipe 1 and an outer pipe 2, an atomizing tank 3 and a blower 4. A heat source fluid flowing in from an entry section 5 of the outer pipe 2 flows in the direction of small arrows between the inner pipe 1 and the outer pipe 2, and a misty air current consisting of air sucked in by the blower 4 and extremely fine fluid drops atomized by an ultrasonic oscillator 6 of the atomizing tank 3 flows in the direction of large arrows in the inner pipe 1. And therefore, the extremely fine water drops in the misty air current collide with wall of the inner tube 1 and become evaporated by absorbing heat from the heat source fluid. As a large volume of evaporating latent heat is utilized by atomizing an extremely small volume of liquid, it is possible to obtain large cooling effect with small fluid consumption. And since the liquid drops are evaporated, no consideration is required for disposal of the misty air current. And therefore, a heat exchanger becomes able to be made smaller.