A thermoelectric conversion device includes: an insulating layer; and a thermoelectric conversion module disposed on the insulating layer. The thermoelectric conversion module has a first thermoelectric conversion region and a second thermoelectric conversion region. The first(second) thermoelectric conversion region includes one or two or more thermoelectric conversion elements, a first(third) connection electrode, and a second(fourth) connection electrode. The thermoelectric conversion elements of the first(second) thermoelectric conversion region are electrically connected to the first(third) connection electrode and the second(fourth) connection electrode and located on an electric path connecting these connection electrodes. Each of the thermoelectric conversion elements includes a thermoelectric converter. The thermoelectric converter of at least one of the thermoelectric conversion elements has a phononic crystal layer having a phononic crystal structure including a plurality of regularly arranged through holes. A through direction of the plurality of through holes in this crystal structure is substantially parallel to a direction perpendicular to a principal surface of the insulating layer.