To provide an EGR cooler of an internal combustion engine that can suppress an increase in flow passage resistance of an EGR gas and a cooling water and improve the conversion efficiency of thermoelectric elements.

An EGR gas flow passage 21 is formed in a cylinder shape with a square cross section, and thermoelectric elements 23 are provided on four outer surfaces of the EGR gas flow passage 21. The EGR gas flow passage 21 is encircled by a cooling water flow passage 22 that is formed cylindrical along the EGR gas flow passage 21, so as to circulate a cooling water therein. Thus, while the high-temperature end face 23a and low-temperature end face 23b of the thermoelectric elements 23 are ensured enough in areas, an increase in flow passage resistance due to the separation or joint of flow is suppressed and the EGR gas and cooling water are smoothly circulated, thereby permitting efficient heat reception and heat radiation of the thermoelectric elements 23.