To eliminate the backlash of a differential gear device without affecting the positional relationship (tooth contact) between a worm wheel and a worm gear.

In the steering angle ratio variable steering device using a differential gear device in which first and second bevel gears 12, 14 form input shafts, and a differential gear case 31 having third and fourth bevel gears 21, 22 and a worm wheel 33 forms a steering angle increasing/decreasing shaft, spindles 41, 42 of the third and fourth bevel gears are movably supported in the direction A by long holes 36, 37 in the differential gear case 31, a housing 51 forms first and second housings 52, 55 to be screwed to each other by screw parts 54a, 54a, and the second housing is screwed into the first housing. The third and fourth bevel gears are pushed by the first bevel gear and moved and pressed against the second bevel gear. In this backlash adjustment, the third and fourth bevel gears are not moved in the axial direction, and the positional relationship between the worm wheel and the worm gear is not affected thereby.