To provide a moisture proof main unit for a storage battery which is not exploded even under an internal pressure and in which the storage battery is never contaminated by air.

This main unit has two plates 20 and 21 to be maintained at specified positions by a band 22 having somewhat elasticity. A tightening part 23 can be slid between the plates 20 and 21 not so as to cover an opening 25. Thereby, a relief valve against a too high pressure is obtained. To avoid excessive displacement of this part of the band 22, a lug 30 for avoiding the band from being protruded beyond the plates 20 and 21 is provided.