A porous electrolytic solution reservoir which is capable of being impregnated with an electrolytic solution is provided in an exterior case so as to make contact with a separator. The average diameter of the pores in the electrolytic solution reservoir is greater than the average diameter of pores in the separator. The electrolytic solution reservoir is impregnated with a predetermined amount of electrolytic solution, so that the occupation ratio of electrolytic solution within the pores in the separator becomes 50% or more when fully charged, and the occupation ratio of electrolytic solution within the pores in the electrolytic solution reservoir becomes 100% or less when fully discharged.