To provide a reasonably constituted heat exchanger.

In the heat exchanger provided with a core 200 integrated with flat tubes 210 and corrugated fins 220, and tanks 300 for inserting the terminal of the tubes to effect heat exchange between a medium flowing through the tubes and heat transferred to the core, the width T of the tube in the flatted direction of the same, the width F of the fins and the outer diameter Ko of the tank are in the relation of T<F≤Ko. In this case, a plurality of cores are provided in parallel and a distance D between fins of respective cores is specified so as to be not bigger than 3.0 mm.