To provide a battery condition monitoring device for accurately obtaining internal resistance of a battery, an open voltage of the battery, a charging rate of the battery and the like.

This battery condition monitoring device for monitoring a battery condition is provided with a microcomputer 2 having an internal resistance calculation part 4 where, when it is determined that battery discharge at a certain level or higher has occurred or when it is determined that a predetermined change other than that (for instance, a variation of a battery liquid temperature above a certain level, a variation of a battery voltage value at a certain level or higher or battery charge/discharge at a certain level or higher since previous battery internal resistance update) has occurred in the battery, the internal resistance of the battery is calculated by using each of them as a trigger.