To provide a miniaturized fuel supply device capable of realizing large reduction of CO2, improvement of fuel consumption and clean-up of exhaust gas.

The fuel supply device 10 supplies mixed fuel 55 in which hydrocarbonaceous fuel is added with oxygen-containing compound fuel to a spark ignition type internal combustion engine 1. The fuel supply device 10 includes a first system 51, and a second system 52 which are arranged on the downstream side of a mixed fuel tank 40 storing the mixed fuel 55 therein. The first system 51 is provided with a separation membrane 41 for separating the mixed fuel 55 into hydrocarbon-rich fuel 58 and ethanol-rich fuel 56 as oxygen containing compound-rich fuel. The second system 52 directly supplies the mixed fuel 55 to a main combustion chamber 18 or a sub-combustion chamber 370 of the internal combustion engine 1.

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