To provide a detector which detects disconnection of a battery pack with a simple configuration, using a circuit that detects an overcharging and an over-discharging of a cell.

When a signal (H level) diagnosing any failure is output from a charge-and-discharge control circuit 5, MOS transistors Q1, Q3 to Qn-1 corresponding to cells s1, s3 to sn-1 are forced to turn off, while MOS transistors Q2, Q4 to Qn corresponding to cells s2, s4 to sn are forced to turn on. At this time, if disconnection has been caused between a positive electrode terminal of the cell s2 and a detection terminal C2, the voltage of the detection terminal C2 becomes equal to a voltage of the negative electrode terminal of the cell s2. With such constitution, an abnormality detection circuit b2 outputs a signal indicating that a voltage between terminals C1 and C2 is abnormal, while an abnormality detection circuit b3 outputs a signal indicating that a voltage between terminals C2 and C3 is abnormal, by which the disconnection is detected.

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