PURPOSE: To facilitate operation, washing and maintenance and to enable highly-precise measurement of the specific gravity by a method wherein a change in the reflectivity of a prism face due to a change in the refractive index of a measuring liquid is determined on the basis of measurement of the quantity of sensed light of a measuring light-sensing part.

CONSTITUTION: A prism 10 has a prism face 8 slanting at an angle of 30°, for instance, to the optical axis X of a measuring light-emitting diode 14 intersecting a plane of incidence perpendicularly and a total reflection face 30 slanting at an angle of 60°, for instance, to the optical axis Y of a measuring photosensor 16. A bundle of rays emitted from the diode 14 is partly reflected on a critical plane on the prism face 8 and partly transmitted therethrough. The ratio between the quantity of the reflected light and the quantity of the transmitted light changes in relation to the refractive index of a liquid in which the prism face 8 is immersed, according to the rule of a critical angle. The reflected light by the prism face 8 is then totally reflected by the total reflection face 30 and reaches the sensor 16. A detected value of the sensor 16 is corrected by outputs of a reference photosensor 20 and a temperature sensor 24, the refractive index is computed by a prescribed arithmetic formula and the specific gravity is computed from the refractive index.