PURPOSE: To eliminate self-discharge and restrictions on battery composition materials in a metal-bromine secondary battery of an electrolyte circulating type by adding tertiary alkylamine and acid to a positive electrolyte composed of a metal bromide aqueous solution.

CONSTITUTION: Tertiary alkylamine and acid are added to a positive electrolyte composed of a metal bromide aqueous solustion so that the tertiary alkylamine is dissolved in the electrolyte. The electrolyte in a negative electrolyte storage tank 6 is circurated into a negative chamber 2 by a pump 9, and the electrolyte in a positive electrolyte storage tank 7 is circulated into a positive chamber for charging a battery. During this time, bromine deposited in the negative chamber 2 is either dissolved in the electrolyte, or is reacted with the tertiary alkylamine to generate bromine complex and is settled at the bottom of the tank 7, thus eliminating problems of self-discharge or corrosion. During discharging, the bromine complex and the electrolyte from a bypass 11 which does not include it are sent into the negative chamber 2 for discharge.