A battery monitoring device includes: a current detection unit that detects an electric current flowing through a battery; a voltage detection unit that detects a voltage between both ends of the battery; a temperature detection unit that detects the temperature of the battery; an internal resistance increase rate calculation unit that calculates the internal resistance increase rate of the battery based on the detected electric current, voltage between the both ends, and temperature; and a calculation invalid time setting unit that sets a predetermined calculation invalid time according to the characteristics of the internal resistance increase rate. The calculation results of the internal resistance increase rate is invalidated for an invalid period from the start of charging or discharging of the battery to the lapse of the calculation invalid time.
