To judge a degradation cause by understanding a usage environment condition in which a secondary battery has actually experienced before its degradation occurs.

Various conditions concerned with a secondary battery's performance are detected from a temperature detection means 18, a voltage detection means 16 and a current detection means 17. Experience time, experienced charge, discharge current volume, or condition detection count is integrated and stored in a storage means 15 through a computing means 14. By storing the information to judge a degradation condition of the secondary battery, variation in a usage environment is known to judge the degradation cause.