A transmittance sensor that detects the optical transmittance of biofuel is provided on a fuel tank that stores biofuel to be supplied to an internal combustion engine. An ECU periodically acquires the optical transmittance detected by the transmittance sensor, and calculates the amount of change from the previous value. In addition, the ECU calculates a degradation index value indicating the degree of degradation of the biofuel by accumulating the amount of change.