PURPOSE: To measure the residual capacity precisely by determining the mass of a vibratory pipe vibrating part in the condition that a Zn-Br battery is completely discharged, and therewith calibrating the device.

CONSTITUTION: When measurement of residual capacity is to be made as usual operation, oscillating frequencies (f) of vibratory pipes 46-P, 46-N are sensed, and a calculating device 58 calculates the density (d) of the electrolyte flowing through these vibratory pipes from the sensed frequency (f) and the mass M determined at the previous calibration run. When user gives a command about complete discharge the calculating device 58 gives a complete discharge command to a perfect discharging device as calibrating operation. The complete discharge condition of the battery concerned is sensed with the voltage measured by a voltmeter 80 in the condition that current is supplied to an end electrode 70 by a constant current source 78. When informed of complete discharge condition, the calculating device 58 starts calculating concerning the calibration. Now the mass M of the vibrating part associate with vibratory pipe 46-P, 46-N is determined, and precise measurement of capacity can be performed.