In a memory, the surface temperature and the internal resistance of an assembled battery detected under the condition where a difference between the surface and the internal temperature is within a predetermined value are stored, and an internal temperature diagnosis unit that diagnoses whether or not the internal temperature estimated by an internal temperature estimation unit is correct, detects the internal resistance with an internal resistance calculation unit when the internal temperature estimation unit estimates the internal temperature, searches for an internal resistance corresponding to the surface temperature equal to the estimated internal temperature value from among the stored internal resistances, and diagnoses the estimated internal temperature value based upon the result of comparison of a search result of the internal resistance and the internal resistance detected during internal temperature estimation.