PURPOSE: To improve the precision of specific-gravity measurement by reducing a measured-temperature error by employing structure of high heat transmitting resistance for the support part of a main equipment body.

CONSTITUTION: Main equipment body 18 consists of light emitting diode 9, measuring prism 12, reflecting mirror 13, phototransistor 15, heat-sensing element 16, etc. This main body 18 is supported by battery cover 2 through support part 19. Internally reflected light Li of light Ls from light emitting diode 9 varies with the specific gravity of electrolyte 4. Although this changed quantity of light is used to fine the specific gravity by fixed arithmetic, the reflective index of electrolyte 4 varies with temperature. To prevent the variation, support part 19 is separated from main body 18 and made of a material small in heat transfer coefficient with its sectional area reduced. Consequently, a difference between the measurement temperature of heat sensing element 16 and a liquid temperature is eliminated, and consequently the precision of specific-gravity measurement can be improved.