A metal-halogen secondary battery has a high state of charging and a capability of storing a large electric power at a high density. In the secondary battery the electrolyte pressure in the positive electrode chamber is higher than the electrolyte pressure in the negative electrode chambers. The metal ions in the positive electrode chambers are permeated into the negative electrode chambers and reduced, and the halogen ions in the negative electrode chambers are also permeated into the positive electrode chambers and oxidized thereby contributing to the storage of the electric power.