In this invention, provided separately from a driving battery serving as a power source for a vehicular driving motor is an auxiliary battery serving as an auxiliary power source, said auxiliary battery having an output voltage that is lower than the driving battery, and wherein the magnitude of the terminal voltage and the magnitude of the current flowing therethrough are detected by a sensor unit from a status determination device. An internal resistance calculation unit calculates the internal resistance of the auxiliary battery on the basis of the magnitudes of the terminal voltages and the magnitudes of the currents detected by the sensor unit in: a stable period before the auxiliary is activated and during which the magnitude of the terminal voltage and the magnitude of the current fall within predetermined variation ranges; and a predetermined period since the activation of the auxiliary has been detected.