To prevent rupture of a lead-acid battery for an automobile due to explosion thereof.

An auxiliary tank 2 of a sealed container is installed on the circumferential part excluding the upper surfaces of a storage battery 1 and a lid 6 of the storage battery I, water is poured in advance into the auxiliary tank 2, and the air is left in its upper part. A thin tube 3 connecting between the storage battery 1 and the auxiliary tank 2 is formed, one end of the thin tube 3 is inserted to the prescribed position of a space 7 behind the lid of the storage battery 1, and the other end thereof is inserted to a position reaching the bottom wall 15a of the auxiliary tank 2.