An electric actuator, comprising: a motor part configured to drive upon receiving supply of power; and a motion conversion mechanism part configured to convert a rotary motion of the motor part into a linear motion to output the linear motion, wherein the motion conversion mechanism part comprises: a screw shaft arranged coaxially with a rotation center of a rotor of the motor part; a nut member rotatably fitted to an outer periphery of the screw shaft; and a planetary gear speed reducer configured to reduce a speed of rotation of the rotor and output the rotation, wherein the rotor comprises: a rotor core which holds a rotor magnet; and a hollow rotary shaft, which has the rotor core mounted to an outer periphery thereof, and has the nut member arranged on an inner periphery thereof, wherein the hollow rotary shaft is rotatably supported by rolling bearings (27, 30) arranged at two positions apart from each other in the axial direction, and comprises an inner raceway surface for one of the two rolling bearings (27, 30), wherein the screw shaft is configured to advance toward one side in an axial direction or retreat toward another side in the axial direction in accordance with a rotation direction of the nut member, and wherein the nut member is arranged on an inner periphery of the hollow rotary shaft, and is coupled to an output member of the planetary gear speed reducer so as to be capable of transmitting torque the electric actuator being characterized in that, the output member comprises a cylindrical portion interposed between an inner peripheral surface of the hollow rotary shaft and an outer peripheral surface of the nut member, wherein the planetary gear speed reducer comprises a sun gear press-fitted and fixed to the inner peripheral surface of the hollow rotary shaft, wherein an inner peripheral surface of the cylindrical portion is fixed to the outer peripheral surface of the nut member, and wherein an outer peripheral surface of the cylindrical portion is opposed to the inner peripheral surface of the hollow rotary shaft, and an inner peripheral surface of the sun gear through intermediation of a radial gap. a motor part configured to drive upon receiving supply of power; and a motion conversion mechanism part configured to convert a rotary motion of the motor part into a linear motion to output the linear motion, wherein the motion conversion mechanism part comprises: a screw shaft arranged coaxially with a rotation center of a rotor of the motor part; a nut member rotatably fitted to an outer periphery of the screw shaft; and a planetary gear speed reducer configured to reduce a speed of rotation of the rotor and output the rotation, a screw shaft arranged coaxially with a rotation center of a rotor of the motor part; a nut member rotatably fitted to an outer periphery of the screw shaft; and a planetary gear speed reducer configured to reduce a speed of rotation of the rotor and output the rotation, wherein the rotor comprises: a rotor core which holds a rotor magnet; and a hollow rotary shaft, which has the rotor core mounted to an outer periphery thereof, and has the nut member arranged on an inner periphery thereof, a rotor core which holds a rotor magnet; and a hollow rotary shaft, which has the rotor core mounted to an outer periphery thereof, and has the nut member arranged on an inner periphery thereof, wherein the hollow rotary shaft is rotatably supported by rolling bearings (27, 30) arranged at two positions apart from each other in the axial direction, and comprises an inner raceway surface for one of the two rolling bearings (27, 30), wherein the screw shaft is configured to advance toward one side in an axial direction or retreat toward another side in the axial direction in accordance with a rotation direction of the nut member, and wherein the nut member is arranged on an inner periphery of the hollow rotary shaft, and is coupled to an output member of the planetary gear speed reducer so as to be capable of transmitting torque the output member comprises a cylindrical portion interposed between an inner peripheral surface of the hollow rotary shaft and an outer peripheral surface of the nut member, wherein the planetary gear speed reducer comprises a sun gear press-fitted and fixed to the inner peripheral surface of the hollow rotary shaft, wherein an inner peripheral surface of the cylindrical portion is fixed to the outer peripheral surface of the nut member, and wherein an outer peripheral surface of the cylindrical portion is opposed to the inner peripheral surface of the hollow rotary shaft, and an inner peripheral surface of the sun gear through intermediation of a radial gap.