A power transmission device of a work vehicle has an input shaft, an output shaft, a gear mechanism, and an electric motor. The gear mechanism has a planetary gear mechanism and transmits the rotation of the input shaft to the output shaft. The electric motor is connected to a rotating element of the planetary gear mechanism. The power transmission device is configured to change the rotation speed ratio of the output shaft with respect to the input shaft by changing the rotation speed of the electric motor. A control unit has a target torque determination unit and a target torque correcting unit. The target torque determination unit determines a target torque of the electric motor. The target torque correcting unit corrects the target torque according to a correction torque based on a moment of inertia of the electric motor.