Provided is a transmission control device which can improve drivability in an acceleration section even while taking a reduction of fuel consumption in a deceleration section into consideration. The transmission control device (transmission controller) which controls a transmission in a running control of a vehicle includes a shift timing calculation unit which determines an acceleration position at which the vehicle accelerates on the basis of a target speed pattern generated from external information, a target gear ratio calculation unit which determines a required gear ratio which is required at the acceleration position, and a gear ratio overwriting command unit which outputs a command of overwriting the gear ratio such that the gear ratio at the acceleration position approaches the required gear ratio. In the control of the transmission of the transmission controller, the gear ratio is set to the required gear ratio (target gear ratio) at a predetermined point after the vehicle exits the deceleration section and reaches the acceleration position.