PURPOSE: To obtain an inexpensive sensor having long service life and high reliability by using a commercially available primary cell for charging a sp. gr. sensor.

CONSTITUTION: The sp. gr. sensor having about 100mAh capacity and consisting of a lead electrode and lead dioxide electrode is attached to a lead cell for placing having 1.215 sp. gr., e.g. two manganese dry cells are connected in series, and these are connected to the sp. gr. sensor in the direction for charging said sensor through about 1,000Ω resistance. Then, charging current of about 1mA flows through the electrode of the sp. gr. sensor. A relay operating by a timer is put to the circuit, charging is performed by closing its circuit for 1hr per day, and remaining 23hr is made to opened circuit. The sp. gr. sensor is operated normally even after the lapse of two years.