A transmission controller includes a processor configured to set acceleration/deceleration start timing at which a vehicle starts accelerating or decelerating, based on at least one of a sensor signal representing the situation around the vehicle, the current position of the vehicle, a map including information on a road being traveled by the vehicle, and operation of the vehicle by a driver, downshift a second transmission of a power train including two motors and an engine before the acceleration/deceleration start timing, and control the power train to vary first and second gear ratios of a first transmission so as to keep the RPM of the engine constant. The first transmission is capable of steplessly varying the first and second gear ratios, which are gear ratios between one of the motors and the engine and between the other motor and the engine, respectively.