There is provided a field device performing a communication through a communication line. The field device includes: first circuits involved with a smart communication; second circuits involved with a fieldbus communication; a token detecting circuit that detects a token in the fieldbus communication; and a control circuit. The control circuit is operable to: i) cause the second circuits to operate when the token is detected within a given time by the token detecting circuit; and ii) cause the second circuits not to operate when the token is not detected within the given time by the token detecting circuit.