A controlling device for controlling the transmission ratio of a non-stage transmission in a vehicle. The transmission ratio is controlled according to a forecast output power of the vehicle. The output power is forecast based on an operating condition of the vehicle, such as atmospheric pressure. The control considers various operating parameters. A comparison of a predetermined increases of engine speed or throttle opening with actual ratios of increase of engine speed or throttle opening may be made in controlling the transmission ratio. An uneven road surface is detected and a transmission ratio of the transmission is controlled accordingly to substantially eliminate hunting. An engine speed is controlled so that the engine speed may not exceed a predetermined rotational speed when the transmission gear ratio is fixed. A target engine speed or target transmission ratio is corrected in response to an output signal of a comparison device which compares an actual vehicle velocity with a set vehicle velocity during automatic fixed vehicle velocity running.