A transmission control device controls a CVT installed in a vehicle. The device has, as modes for the CVT, a CVT mode M1 for changing a transmission gear ratio steplessly based on the operating state of the vehicle, and a stepped transmission mode M2 for changing the transmission gear ratio stepwise based on the operating state of the vehicle. The device includes a transmission mode switching control unit (controller) 12 for controlling switching between the CVT mode M1 and the stepped transmission mode M2 based on an actual accelerator opening. The transmission mode switching control unit (controller) 12 performs switching from the CVT mode M1 to the stepped transmission mode M2, if the actual accelerator opening reaches or exceeds a first threshold value T1 set in a plurality of stages so as to increase stepwise in accordance with an increase in a vehicle speed.