A gear ratio control unit is provided to set a target gear ratio on the basis of an operating condition of the vehicle. The gear ratio control unit controls a drive amount of a step motor such that an actual gear ratio converges with the target gear ratio, and when a deceleration of the vehicle exceeds a predetermined threshold, controls the gear ratio by controlling the drive amount of the step motor such that a predetermined gear ratio is maintained. The gear ratio control unit determines the predetermined threshold on the basis of the actual gear ratio upon detection of an operation of a brake.