To calculate the maximum charge/discharge power of a battery accurately even if, for example, an electric car is running at a constant speed or begins to run.

A characteristic formula Wh(P) or Wh-P characteristic table that is a correlation between the amount of discharge power and the maximum discharge power of a battery 11 is calculated in advance, and the maximum discharge power or the maximum charge power is calculated based on the correlation and the integrated amount of discharge power. Especially, the above correlation is corrected by a temperature correction coefficient that is obtained by calculation based on a temperature T of the battery 11 and a deterioration correction coefficient that is obtained by calculation based on the deterioration of the battery 11, thus calculating the maximum discharge power and the maximum charge power accurately.