To provide a transmission ratio variable device allowing improvement of slip resistance against instantaneous impact force during a lock operation and maintaining of steering performance during an abnormality of a differential mechanism.

In this transmission variable device, a lock device includes an auxiliary lock holder 101 formed with an auxiliary engaging groove engageable with an engaging pawl 60 on an outer peripheral surface. The auxiliary lock holder 101 is arranged side by side with a lock holder 53 in a shaft direction of a motor shaft and is fixed to be relatively non-rotatable to the motor shaft. The lock device includes a lock arm 54 to be movable between a first engaging position where the lock arm 54 is capable of being engaged to an engaging groove and the auxiliary engaging groove and a second engaging position where the lock arm 54 is capable of being engaged to only the engaging groove. In the lock device, a driving mechanism 55 moves the lock arm 54 to the second engaging position after the lock arm 54 restricts rotation of the motor shaft in a state of the first engaging position during the lock operation for restricting the rotation of the motor shaft.