PURPOSE: To provide an optical fuel specific gravity sensor capable of correctly detecting specific gravity of engine supply fuel by using an optical means.

CONSTITUTION: An optical fuel sensor 25 is furnished with a position detection light receiving device 39 to detect change of an optical path accompanying change of a refractive index of fuel. Output from the position detection light receiving device 39 is processed through a refractive index detection means 40, a specific gravity detection means 41, a specific gravity correction means 42 and a heavy duty correction computing means 43, and it is used to calculate a heavy duty fuel correction coefficient at the time of fuel supply. Heavy duty fuel correction is carried out at the special time of driving such as at the time of cold driving. A control unit is furnished with a map to show relations between fuel specific gravity and fuel temperature, and carries out correction in accordance with the fuel temperature obtained by a temperature sensor. In this case, the higher the fuel temperature goes up, the more correction increases.