In a fuel supply device for separating raw fuel into high-octane fuel and low-octane fuel and supplying the fuel, to arrange the structural components compactly and to facilitate sealing against fuel vapor, the fuel supply device (1) includes: a raw fuel tank (2) for storing raw fuel; a separator (6) provided inside the raw fuel tank to separate the raw fuel into high-octane fuel that contains a greater amount of components with high octane numbers than the raw fuel and low-octane fuel that contains a greater amount of components with low octane numbers than the raw fuel; and a high-octane fuel tank (5) provided inside the raw fuel tank to store the high-octane fuel separated from the raw fuel by the separator.