PURPOSE: To make operation simple and to prevent oil leakage by accommodating insulating oil increased according to the generation of gas during charging in an insulating oil reservoir through a connecting hose, and returning the insulating oil in the reservoir to a battery system.

CONSTITUTION: Before charging, gas exhaust auxiliary equipment 1 is connected to an immersed-in-oil battery system through a connecting hose 6. With the progress of charging, gas generated in unit cells 12 surfaces as bubbles, and reaches an insulating oil reservoir 2 through a gas-liquid separating plug 16 and the connecting hose 6, then exhausted outside from the opening 3 of the reservoir 2. After completion of charge, a compressor is connected to an opening coupler 8 of the opening 3, and the reservoir 2 is compressed, then recovered to the normal pressure. By repeating the compression and the recovery, residual gas in the plates and reparators of unit cells 2 is exhausted outside, the insulating oil 15 in the reservoir 2 is returned to a cell accommodation tank 10. Therefore, no oil leakage is generated.