10 and bonded to the same as shown in fig3 thus an optical unit of the light reflecting type of optical sensor similar to that shown in fig3 was prepared to improve the measuring accuracy of the sensor the output of the photo detector 7 was separated into ac and dc components and the ac component was electrically divided by the dc component as described with respect to example 1 the prepared optical sensor was placed in a thermostat and ac 50 v 60 hz was applied to the electrooptical crystal 3 while the temperature of the sensor was varied between 20 u00b0 c and 80 u00b0 c a measurement of an output signal fluctuation of the sensor was 4 in the meantime an optical sensor was prepared as shown in fig1 a which uses a catio 3 polycrystal substrate 20 which has a constricted portion in the form of a rectangular protrusion 28 defined by and between two parallel rectangular grooves 29 formed in the upper surface of the substrate 20 such that the rectangular protrusion 28 is left in a central part of the substrate 20 the grooves 29 have dimensions as indicated in fig1 a the crystal 23 is bonded to the upper surface of the substrate 20 by the adhesive layer 19 as shown in fig1 b thus the rectangular.