A battery pack which stably implements a protection function of a secondary battery mainly under software control and reduces a circuit installation area and parts costs. The battery pack has a discharge current cutoff means and a charge current cutoff means for selectively cutting off discharge and charge currents of the secondary battery. The battery pack has a protection process means for controlling operations of the discharge current cutoff means and the charge current cutoff means in accordance with an inter-electrode voltage between positive and negative electrodes of the secondary battery. The battery pack executes an initialization process for the protection process means at a timing when a power supply voltage supplied to the protection process means reaches lowest voltage to start the protection process means. Further, the battery pack sets a threshold voltage needed for operation control of the discharge current cutoff means and the charge current cutoff means to start the protection process at another timing when the power supply voltage reaches voltage that is higher than lowest voltage and is smaller than or equal to a voltage for stable driving.