Variable transmission ratio device and steering device

Tooth jumping variable transmission ratio device that can be effectively suppressed between the flexible spline and the circular spline caused by steering operation at the time of locking operation is provided. Variable transmission ratio device 1 includes a harmonic drive gear mechanism 10 for transmitting and plus a rotation based on driving by the motor to the rotation of the input shaft 3 based on the steering operation to the output shaft 4, while accommodating the wave gear mechanism 10 and the motor 2 comprises a housing 6 which is fixed to the non-rotating portion, and a locking device 21 for locking the housing 6 and the motor shaft 15 in a relatively non-rotatable manner. Of the circular spline 11, 12 which constitute the wave gear mechanism 10, the number of teeth of the circular spline 12 which is connected to the output shaft 4 is set larger than the number of teeth of the circular spline 11 which is connected to the input shaft 3 .