A control apparatus for an internal combustion engine, includes circuitry. The circuitry is configured to control a ratio of an amount of low octane number fuel to be supplied to a cylinder to a total amount of the low octane number fuel and a high octane number fuel to be supplied to the cylinder in order to control an overall octane number of fuel to be supplied to the cylinder. The high octane number fuel has a second octane number higher than a first octane number of the low octane number fuel. The circuitry is configured to calculate a maximum octane number of the fuel to be supplied into the cylinder. The circuitry is configured to restrict a power generated by the internal combustion engine based on the maximum octane number.