A plurality of voltage switching circuits 41 are provided in a switching unit 40 to relatively change a reference voltage to plural levels, thus detecting a spontaneous change of the reference voltage. A range of relative change in the reference voltage by the plurality of voltage switching circuits 41 is set to a usage voltage range 81 or 82 as a part of a total voltage range 80 of each of cells 1a to 1d. This eliminates the need to provide voltage switching circuits 41 required to relatively change the reference voltage over the total voltage range of each of the cells 1a to 1d, making it possible to minimize the number of the voltage switching circuits 41 for each of the cells 1a to 1d. Thus, it is possible to prevent the size of a battery pack control apparatus 2 from increasing.
