A battery pack has a secondary battery, first and second switch devices provided between the secondary battery and a load or a charging unit, a protection circuit detecting an overcharge, an overdischarge or an overcurrent of the secondary battery and controlling turning on and off of the first and second switch devices, a series circuit of a resistor and a thermistor disposed in the vicinity of the secondary battery and connected in parallel with the secondary battery, a comparator which compares a voltage at a connection point between the thermistor and the resistor with a reference voltage corresponding to a predetermined temperature inside the protection circuit, and a third switch device connected between the resistor and a negative electrode of the secondary battery. The protection circuit turns off the first switch device and the third switch device when an overdischarge of the secondary battery is detected.