A hybrid work vehicle is provided with an internal combustion engine for supplying driving force to a running device and a work implement via a transmission shaft, a motor generator for assisting the internal combustion engine by outputting power to the transmission shaft, and a battery for receiving charging electricity from the motor generator and giving driving electricity to the motor generator. A load information generation unit generates load information indicating a rotational load received by the internal combustion engine when in a constant-speed control mode for maintaining a constant number of revolutions of the internal combustion engine, based on an input parameter, and an assistance calculation unit calculates an amount of assistance for internal combustion engine by the motor generator, based on the load information when in the constant-speed control mode.