A battery ECU includes a central processing unit (CPU). The CPU calculates plural deterioration degrees based on data input from a thermistor, an ammeter, and a voltmeter which detect state quantities of a nickel-hydrogen battery, and reads, from a memory, contribution degrees each of which corresponds to each of the calculated plural deterioration degrees. Then the CPU calculates a total deterioration degree of the nickel-hydrogen battery based on the deterioration degrees and the contribution degrees, calculates the contribution degrees based on the calculated total deterioration degree, and stores the contribution degrees in the memory.