A field device has a primary power supply generating circuit that generates a primary power supply from an electric current that is supplied through a pair of electric wires from a higher-level system, and a calculation processing portion and various functional circuit portions, which operate based on a supply of an operating power supply electric current produced from the primary power supply. The field device also has an operating power supply electric current supplying unit that supplies the operating power supply electric current to a calculation processing portion with maximum priority. The calculation processing portion receives the operating power supply electric current supplied with maximum priority, clears a self-reset operation after starting up itself, and then directs sequentially, following a predetermined sequence, supply of the operating power supply electric current to each of the various functional circuit portions.