To provide a motor-driven chuck device capable of improving durability against rotation of a main spindle and rotating the main spindle at high speed by transmitting driving force of an engagement member to a draw tube via an increased force transmission mechanism.

This device has a chuck provided at a tip of the main spindle 5 and provided with an opening/closing mechanism for opening/closing a gripping claw, a driving motor 1 for driving the gripping claw to open/close, an advancing/retracting member 4 provided to be advanced/retracted in an axis direction of the main spindle within a through hole of the main spindle and operating the opening/closing mechanism, an engaged part 77 for transmitting driving force of the axis direction of the main spindle to the advancing/retracing member 4, an engagement member 3 engaged to the engaged part 77 to transmit driving force of the axis direction of the main spindle to the engaged part 77, the increased force transmission mechanism 7 provided between the engaged part 77 and the advancing/retracting member 4 to transmit the driving force of the axis direction of the main spindle impressed on the engaged part 77 to the advancing/retracting member 4 by increasing the driving force, and a conversion mechanism for converting a rotary motion of the driving motor 1 into a linear motion of the axis direction of the main spindle of the engagement member 3.