The temperature of a battery cell measured by a temperature measurement means is stored into a measured temperature history storage means for storing a history of temperature. An in-cell temperature inference means extracts a minimum temperature from the history of a predetermined time from the measured temperature history storage means, and infers the minimum temperature as a current temperature within the battery cell. A remaining power calculation means calculates remaining battery power based on the inferred temperature within the battery cell. Instead of directly using the measured temperature for the calculation of the remaining battery power, the inferred temperature within the battery cell is used, whereby even if the measured temperature rises rapidly, it is possible to prevent the phenomenon instantaneous increase of the calculated remaining battery power.