A secondary battery of the present invention a heat-sensitive element which varies in resistance value due to change of temperature is not placed on a path through which a charge or discharge current flows into or out of a storage battery. Accordingly, when a main switch element generates heat due to an overcurrent, a change in resistance value of the heat-sensitive element resulting from the heat will allow an auxiliary switch to be turned ON, thereby causing first and second auxiliary fuses to be blown. The charge or discharge current causes no power consumption at the heat-sensitive element, thereby providing high efficiency. The heat-sensitive element can be reduced in current capacity for use and it is suitable for compact design.