PURPOSE: To reduce the size in the thickness direction of the titled detector so that the thickness thereof is reduced and the structure over the entire part is made into the smaller size by housing a light receiving element and light emitting element on the same size and adequately setting the spacing therebetween and the size thereof.

CONSTITUTION: The light from the light emitting element 5 passes a mold layer M, is reflected on the boundary part between flint glass 3 and mixed fuel, returns backward in the flint glass 3 and the mold layer M and is entered to the light receiving element 6. The refractive index of the mixed fuel varies at every fluctuation in the mixing ratio of gasoline and methanol at this time. Since the critical angle of the light from the light emitting element 5 with respect to the flint glass 3 is thereby changed, the mixing ratio of the gasoline and methanol can be determined from the quantity of the incident light on the light receiving element 6. The size of the spacing between the light emitting element 5 and the light receiving element 6 and the size of the light receiving element are set at the relation at which the reflected light from the critical angle of the glass with one fluid to the critical angle of the glass with the other fluid is received.