To provide a transmission ratio variable device for suppressing excessive impact torque at the time of locking and deterioration in the positional accuracy of locking.

The inertia of a rotary member 35 rotated relatively to a first steering shaft 14 by the driving operation of a motor 34 is not lower than first inertia equivalent to an inertia value at which rotating speed of a lock holder 71 arranged at the outer periphery of one end 34a of a motor shaft 34A, i.e. the rotating speed of the rotary member 35 influences the positional accuracy of locking and is not higher than second inertia equivalent to an inertia value at which excessive torque is generated at the lock holder 71 in an engaged state.