An automatic transmission controller calculates a predicted vehicle speed with reference to a measured vehicle state variable, and calculates a target transmission ratio with reference to the predicted vehicle speed, when a sensor input signal is normal which carries information about the measured vehicle state variable. The automatic transmission controller calculates the predicted vehicle speed with reference to a first alternative value instead of the measured vehicle state variable, and calculates the target transmission ratio with reference to a second alternative value instead of the predicted vehicle speed, when the sensor input signal is abnormal. The automatic transmission controller restarts calculation of the predicted vehicle speed with reference to the measured vehicle state variable, when the sensor input signal becomes normal, and restarts calculation of the target transmission ratio with reference to the predicted vehicle speed, when a predetermined condition is satisfied after the sensor input signal becomes normal.