A lock device of output shaft capable of stopping the output shaft quickly when the output shaft is controlled to stop, and locking to prevent the output shaft from rotating when the output shaft side is stopped against the drive side. The output shaft is formed by connection of drive shaft and driven shaft. Play for a specific angle is formed in mutual rotating direction, at a junction of connection of the drive shaft and driven shaft. A holding plate is fixed to the driven shaft side confronting the drive shaft in the axial center direction. An annular lock ring forming a lock pawl on the outer circumference is held movably to concentric position and eccentric position, at the holding plate side. A lock operation member is provided in the drive shaft confronting the lock ring, for allowing the lock ring to move to the concentric position when the play at the junction of the drive shaft and driven shaft is eliminated by initial rotation of rotation of drive shaft, and allowing to move to the eccentric position when rotation of the drive shaft is stopped to return a play at the junction by reaction. A internal ring forming a pawl for locking mutual rotation by mutually engaging with the lock pawl on the outer circumference of the lock ring when the lock ring is moved to the eccentric position is formed oppositely on the outer circumference of the lock ring.