In an internal combustion engine using hydrocarbon fuel and hydrogen gas as fuels and including a fuel injection device that injects liquid fuel, a hydrogen-blended fuel in which hydrogen gas is contained in the form of minute bubbles in a liquid hydrocarbon fuel, that is supplied to the fuel injection device. The hydrogen-blended fuel is stored in a fuel tank. Hydrogen gas that has escaped from the hydrogen-blended fuel in the fuel tank is fed to a minute-bubble producing device, which forms the fuel gas into minute bubbles and mixes the minute bubbles back into the hydrogen-blended fuel.