PURPOSE: To perform battery control automatically by measuring a specific gravity through the fact that a sound velocity in a electrolyte changes with the specific gravity and the temperature of the electrolyte and also measuring a liquid level with the required time of the sound wave for getting to the liquid surface and the sound velocity.

CONSTITUTION: An ultrasonic type specific gravity and liquid level detector of embed type 11 is provided in the electrolyte of a battery 10, and the output thereof is connected to a sound velocity temperature change-over device 14, an ultrasonic oscillation measuring instrument 16 and an artithmetic unit 15 through the first and the second transmission lines, 12 and 13. The temperature-sound velocity detecting element of said detector 11 consists of an ultrasonic vibrator 111, a reflection member 114 and a semiconductor rectifying element 116 for a temperature sensor. Further, its detecting element for the liquid surface level is composed of an ultrasonic vibrator 119 applying a liquid surface as a reflection surface and rectifying element 116 for temperature sensor. Accordingly, the specific gravity and the liquid surface level calibrated for the temperature can be automatically obtained, by measuring the sound velocity and the temperature in the electrolyte, thereby being able to attempt the automation of the battery control.