An abnormality diagnostic device is configured to diagnose an abnormality in a battery pack having a plurality of cells connected in series. Each of a plurality of diagnostic voltage detecting circuits is configured to detect one of a voltage across a corresponding one of discharging circuits. An abnormality diagnostic control section is configured to perform a first diagnostic operation in which the switches corresponding to alternate ones of the cells are turned on and a second diagnostic operation in which all of the switches are turned on, and to determine whether a break exists in an electrical connection or an abnormality exists in one of the switches based on the voltages detected by the diagnostic voltage detecting circuits during the first diagnostic operation and the voltages detected by the diagnostic voltage detecting circuits during the second diagnostic operation.