To provide a protection circuit capable of preventing self-heating in discharging by performing temperature protection on a secondary battery with high precision only when a charger is connected.

The protection circuit includes: a series circuit of a thermistor R13 and a resistance R14 which are arranged nearby a secondary battery 12 and connected to the secondary battery in parallel; a connection detection circuit 32 and a resistance R15 which are connected to a charging device 33 in an open-drain manner through two power supply terminals 13, 14 and one external terminal 32; a comparator 21 which compares the voltage at a connection point between the thermistor R13 and the resistance R14 with a reference voltage corresponding to a predetermined temperature V1; gate circuits 30, 31 which make the output signal of the comparator effective with the output of the connection detection circuit based upon the output of the charging device 33 to the external terminal 32; and a logic circuit 19 which controls switch elements M11, M12 based upon outputs of the gate circuits. The logic circuit turns off the switch elements based upon the output signals from the gate circuits to the comparator 21 when the temperature of the secondary battery exceeds the predetermined temperature.