In the case where a low-pressure-side supercharging pressure acquired value corresponding to the pressure of inlet gas at the outlet of a low-pressure compressor operated in a twin supercharging mode is less than a target supercharging pressure in a single supercharging mode, changeover from the twin supercharging mode to the single supercharging mode is prohibited. Meanwhile, in the case where the low-pressure-side supercharging pressure acquired value is equal to or greater than the target supercharging pressure, the changeover from the twin supercharging mode to the single supercharging mode is permitted. Alternatively, in the case where a supercharging pressure during operation in the single supercharging mode is less than a target value of the pressure of supply gas at the outlet of the low-pressure compressor in the twin supercharging mode, changeover from the single supercharging mode to the twin supercharging mode is prohibited. Meanwhile, in the case where the supercharging pressure is equal to or greater than the target value, the changeover from the single supercharging mode to the twin supercharging mode is permitted.