An internal combustion engine fuel supply system includes a first fuel tank, a separator, and circuitry. The first fuel tank is to store fuel. The separator is connected to the first fuel tank to separate the fuel supplied from the first fuel tank into a high octane fuel and a low octane fuel which are to be supplied to an internal combustion engine. The high octane fuel has a first octane number. The low octane fuel has a second octane number lower than the first octane number. The circuitry is configured to determine whether fuel has been supplied to the first fuel tank, and to operate the separator to separate the fuel into the high octane fuel and the low octane fuel when it is determined that fuel has been supplied to the first fuel tank.