To protect battery elements by preventing not only a battery pack of a battery power source in an electric propulsion system including a hybrid power source including the battery power source composed by forming the battery pack by connecting a plurality of battery elements in series to and in parallel to one another, and connecting a plurality of the battery packs in parallel to one another, and a power generation device for charging the battery power source, but also battery element single bodies constituting the battery packs, from coming into an overcharged state by abnormal voltage drop, and to prevent life of the battery power sources from lowering.

In a mode in which an electric propulsion device is operated by supplying power to the electric propulsion device 3 only from a battery power source 12 of a hybrid power source 1, voltages of battery element single bodies constituting the battery power source (cell voltages) are individually monitored, and, when any cell voltage of the battery element single bodies drops to equal to or below a preset lower-limit value of the cell voltage, the electric propulsion device is operated by decreasing the speed of the electric propulsion device to prevent the cell voltage from dropping below the lower-limit value.