An electric actuator, comprising: a driving motor; a motion conversion mechanism configured to convert a rotary motion of the driving motor to a linear motion in an axial direction parallel with an output shaft of the driving motor; a transmission gear mechanism configured to transmit a driving force from the driving motor to the motion conversion mechanism; and a speed reduction mechanism configured to reduce a speed of the rotary motion of the driving motor, and output the rotary motion reduced in speed to the transmission gear mechanism, wherein a side of one end portion of a rotation shaft of a gear of the transmission gear mechanism is rotatably supported by a bearing, and a side of another end portion of the rotation shaft of the gear is rotatably supported by the output shaft of the driving motor, characterised in that the speed reduction mechanism comprises a planetary-gear speed reduction mechanism, comprising: a sun gear integrally mounted to the output shaft of the driving motor; a ring gear arranged on an outer peripheral side of the sun gear; a plurality of planetary gears, which are arranged between the sun gear and the ring gear, and are configured to mesh with the sun gear and the ring gear; and a planetary gear carrier, which is configured to rotatably hold the plurality of planetary gears, and is integrally mounted to the rotation shaft of the gear of the transmission gear mechanism, and wherein the output shaft of the driving motor passes through the sun gear of the planetary-gear speed reduction mechanism, and is inserted into a shaft hole formed in the rotation shaft of the gear so as to be capable of relatively rotating. a driving motor; a motion conversion mechanism configured to convert a rotary motion of the driving motor to a linear motion in an axial direction parallel with an output shaft of the driving motor; a transmission gear mechanism configured to transmit a driving force from the driving motor to the motion conversion mechanism; and a speed reduction mechanism configured to reduce a speed of the rotary motion of the driving motor, and output the rotary motion reduced in speed to the transmission gear mechanism, wherein a side of one end portion of a rotation shaft of a gear of the transmission gear mechanism is rotatably supported by a bearing, and a side of another end portion of the rotation shaft of the gear is rotatably supported by the output shaft of the driving motor, characterised in that the speed reduction mechanism comprises a planetary-gear speed reduction mechanism, comprising: a sun gear integrally mounted to the output shaft of the driving motor; a ring gear arranged on an outer peripheral side of the sun gear; a plurality of planetary gears, which are arranged between the sun gear and the ring gear, and are configured to mesh with the sun gear and the ring gear; and a planetary gear carrier, which is configured to rotatably hold the plurality of planetary gears, and is integrally mounted to the rotation shaft of the gear of the transmission gear mechanism, and wherein the output shaft of the driving motor passes through the sun gear of the planetary-gear speed reduction mechanism, and is inserted into a shaft hole formed in the rotation shaft of the gear so as to be capable of relatively rotating.