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. <EOS>