A thermoelectric generator system according to this disclosure includes a thermoelectric generator unit which performs thermoelectric generation using first and second heat transfer media at different temperatures. The unit includes a tubular thermoelectric generator which generates electromotive force in its axial direction based on a temperature difference between its inner and outer surfaces. The generator system further includes a flow rate control system which controls the flow rate of at least one of the first heat transfer medium flowing through a flow path defined by the inner surface and the second heat transfer medium in contact with the outer surface by reference to either information about an operation condition of the generator system or a preset target power output level.