To change a reforming condition of the fuel properly in accordance with the operating condition of the engine in a system to reform the fuel by casting the energy into the fuel for decomposing hydrocarbon molecules in the fuel.

An input energy calculation part 41 selects a proper reforming mode to the engine operating condition among a first reforming mode (the fuel not reformed in the mode), a second reforming mode (in the mode the fuel is reformed by decomposing hydrocarbon molecules of olefin series having many carbon atoms contained in the fuel), and a third reforming mode (in the mode, the fuel is reformed by decomposing all hydrocarbon molecules in the fuel), and calculates an input energy amount required to achieve the selected reforming mode. An input energy control part 42 controls a driving voltage of an ultrasonic radiation device 40 so as to achieve the required input energy amount. Thereby the reformed condition of the fuel is changed according to the energy amount by changing the energy amount to be cast into the fuel in accordance with the engine operating condition.