To provide a heat exchanger easily constructed, having high heat exchange efficiency and capable of using various natural thermal energy.

The heat exchanger 1 comprises a columnar body having a gas flow passage 2 inside and is provided with an inflow port and an outflow port for making gas flow. At least part of a peripheral wall of the columnar body comprises a metallic heat exchange board 3. The inner face of the heat exchange board 3 faces the gas flow passage 2, and inner heat transfer members 4 in a plurality of rows extended in parallel along the gas flow passage 2 are erected on the inner face. The inner heat transfer members 4 are tabular metals or metallic bars continuously erected along the rows.