In a battery pack including a protection circuit, a series circuit of a thermistor and a resistor is disposed in a vicinity of a secondary battery and connected in parallel with the secondary battery. A connection detecting circuit is disposed to detect connection of a charging device to the battery pack. A comparator is disposed to compare a voltage of a junction point of the thermistor and the resistor with a reference voltage corresponding to a predetermined temperature. A gate circuit is disposed to activate an output signal of the comparator only when the connection of the charging device is detected. The protection circuit is arranged so that, when a temperature of the secondary battery exceeds the predetermined temperature, a switch element is turned off in accordance with the output signal of the comparator activated.