A positioner operating mode identifying function is provided in a calculating unit. A pressure value of an amplified pneumatic pressure signal, from a single-mode pilot relay, is detected by a pressure sensor and sent to the calculating unit. The calculating unit, through the positioner operating mode identifying function, monitors a change in the amplified pneumatic pressure signal corresponding to a change in the control signal, and if the direction of change in the control output and the direction of change of the amplified pneumatic pressure signal are the identical direction, identifies the operating mode of the positioner as the positive operating mode. The operating mode of the positioner is identified as the negative operating mode if the direction of change in the control output and the direction of change of the amplified pneumatic pressure signal are the opposite directions. The identification results displayed on a screen of a display unit.