A control device for controlling a transmission configured such that when the speed change mechanism performs switching to a shift speed with a lower speed ratio in a negative torque prediction established state in which predicted input torque is negative, the predicted input torque being a predicted value of input torque input to the input member a predetermined determination reference time later, and being derived on the basis of variations in the input torque, special speed change control is executed in which a disengagement hydraulic pressure is lowered to cause a disengagement element to slip, and the disengagement element is maintained in a slipping state over an entire speed change process, which extends from a time point when the disengagement element starts slipping to a time point when a rotational speed is synchronized with a rotational speed of the input member.