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 (22u, 22v, 22w) in multiple-phase connection; a movable part that applies a driving force to at least one of a rudder surface (2c, 2d) of a tailplane of the airplane, a movable surface (2a, 2b) of a main wing, a landing gear (2f, 2g) that supports wheels for landing, and a liquid pump (2j, 2k) 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 (28u, 28v, 28w) for detecting the phase current of the respective phases flowing in the coil (22u, 22v, 22w) 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 (22u, 22v, 22w) in multiple-phase connection; a movable part that applies a driving force to at least one of a rudder surface (2c, 2d) of a tailplane of the airplane, a movable surface (2a, 2b) of a main wing, a landing gear (2f, 2g) that supports wheels for landing, and a liquid pump (2j, 2k) 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 (28u, 28v, 28w) for detecting the phase current of the respective phases flowing in the coil (22u, 22v, 22w) 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).