To operate a monitoring circuit when charging at least a battery, even if battery voltage is lowered to make the monitoring circuit inoperative, in a battery pack with built-in monitoring circuit operated by making the battery as a power supply.

A microcomputer 32 for monitoring the state of the battery 31 is operated by battery side control voltage Vcc generated by a battery side regulator 33. In the battery side regulator 33, the battery voltage Vbat is inputted via Zener diode D4 and diode D1, and also charger side control voltage Vdd in the charger 20 is inputted via diode D3 and diode D2. As a result, the battery side regulator 33 generates the battery side control voltage Vcc based on the charger side control voltage Vdd to operate the microcomputer 32.

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