The present invention is directed to a device for detecting alcoholic content in a fuel by finding its refractive index, which is of such a construction that a refractive interface for the fuel is formed on the half way of a light transmission body; a light emitting element is provided at one end of the light transmission body, from which incident light is introduced into said refractive interface; and a single dimensional light position sensing detector is provided at the other end of the light transmitting body, into which the light refracted at the refractive interface is introduced; or it is of such a construction that a light emitting element and a single dimensional light position sensing detector are provided at one end of a light transmission body; a refractive interface and a reflecting surface are provided at the other end of the light transmission body, wherein an incident light from the light emitting element, which has entered into the refractive interface and has been refracted there, is reflected at the reflecting surface, after which the reflected light is again caused to enter into the refractive interface, and is subsequently caused to enter into the single dimensional light position sensing detector.