An actuator for an airplane, comprising: a voltage supplier that generates a supply voltage (Vs) based on a power of the airplane and includes a first switcher for lowering the supply voltage (Vs) of the voltage supplier ; an inverter to which the supply voltage (Vs) is supplied; a stator including an armature coil energized and driven by the inverter , wherein the armature coil includes coils in multiple-phase connection; a movable part that applies a driving force to at least one of a rudder surface of a tailplane of the airplane , a movable surface of a main wing, a landing gear that supports wheels for landing, and a liquid pump through magnetic interaction with the stator ; a controller that controls an operation of the inverter in accordance with a command signal from a maneuvering system of the airplane ; an injector that injects a high-frequency signal (Sh) into the armature coil , the high-frequency signal (Sh) having a waveform of one of a trapezoidal wave, a triangular wave, and a staircase wave that varies stepwise between three or more levels, wherein the first switcher lowers the supply voltage (Vs) when the high-frequency signal (Sh) is injected; a current detector that detects a coil current flowing through the armature coil , wherein the current detector includes current detection circuits for detecting the phase current of the respective phases flowing in the coil of the respective phases; and an estimator that estimates a position of the movable part based on a component of the coil current detected by the current detector , the component being related to the high-frequency signal (Sh). a voltage supplier that generates a supply voltage (Vs) based on a power of the airplane and includes a first switcher for lowering the supply voltage (Vs) of the voltage supplier ; an inverter to which the supply voltage (Vs) is supplied; a stator including an armature coil energized and driven by the inverter , wherein the armature coil includes coils in multiple-phase connection; a movable part that applies a driving force to at least one of a rudder surface of a tailplane of the airplane , a movable surface of a main wing, a landing gear that supports wheels for landing, and a liquid pump through magnetic interaction with the stator ; a controller that controls an operation of the inverter in accordance with a command signal from a maneuvering system of the airplane ; an injector that injects a high-frequency signal (Sh) into the armature coil , the high-frequency signal (Sh) having a waveform of one of a trapezoidal wave, a triangular wave, and a staircase wave that varies stepwise between three or more levels, wherein the first switcher lowers the supply voltage (Vs) when the high-frequency signal (Sh) is injected; a current detector that detects a coil current flowing through the armature coil , wherein the current detector includes current detection circuits for detecting the phase current of the respective phases flowing in the coil of the respective phases; and an estimator that estimates a position of the movable part based on a component of the coil current detected by the current detector , the component being related to the high-frequency signal (Sh). <EOS>