An engine system 10A is characterized by having a first electrolyzing device 11A for electrolyzing water into hydrogen and oxygen, an engine 12 which runs on the combustion of a mixed gas of the hydrogen and oxygen generated by the first electrolyzing device 11A, a battery 14 for storing electricity, a boosting coil 16 for boosting the voltage of electricity stored in the battery and supplying the boosted high-voltage electricity to the first electrolyzing device 11A and ignition plugs of the engine 12, and an alternator 15 for generating electricity from the running of the engine and supplying the generated electricity to the battery 14. The engine system can be driven by simply replenishing water without requiring construction of refilling stands or replacement of tanks.