A thermoelectric power generation system 20 includes a heat exchanger 1 having double tubes which are an inner tube 1a and an outer tube 1b, and a thermoelectric power generation module 2 mounted between the inner tube and the outer tube. The thermoelectric power generation module generates thermoelectric power using a temperature difference between a medium inside the inner tube and a medium outside the outer tube, and a highly thermal conductive elastic sheet 3a, 3b is mounted between the thermoelectric power generation module and the inner tube and/or the outer tube in close contact therewith.