<p>A fuel property detecting device having a small simple structure capable of accurately detecting the concentration of the biofuel in a mixed fuel of a hydrocarbon fuel and the biofuel and excellent in durability and reliability. A fuel property sensor (22) comprises a light-emitting device (28) and a light-receiving device (32) both for detecting the light transmittance of the fuel in a fuel passage (26) and a light-emitting device (36) and a position detecting device (38) for detecting the index of refraction of the fuel. Since light transmittance correlates with the RME concentration of the fuel, the RME concentration can be calculated from the detected light transmittance. Since the index of refraction correlates with the cetane number of the fuel, the cetane number can be calculated from the detected index of refraction. The light transmittance is detected by using a light in a wavelength region of 640 to 680 nm.</p>
