PURPOSE: To increase battery capacity by adding a specific ratio of stannic oxide powder in which fluorine is doped in the positive electrode active material of a lead acid battery.

CONSTITUTION: Stannic oxide SnO2 powder, in which fluorine F is doped, is added by 0.5-10% to active material weight to the positive electrode active material of a lead acid battery. All of PbO2 quantity, battery capacity, and the number of a life cycle after formation is increased, by adding SnO2 powder in which F is doped. This is estimated that the PbO2 quantity after formation is increased to increase the battery capacity because of the improvement of the electricity conductivity of the active material due to the addition of the SnO2 powder in which F is doped. Effect, by adding the SnO2 powder, can be obtained sufficiently by making the added quantity of the SnO2 powder, in which F is doped, 0.5-10%, and adding 10% or more can not expect more effect than that.