In an assembled battery monitoring system, a voltage monitoring apparatus monitors voltages of battery cells of an assembled battery. Discharging resistance elements and RC filters are correspondingly coupled between the battery cells and the voltage monitoring apparatus. Discharging switches are disposed inside of the voltage monitoring apparatus correspondingly to the battery cells. The discharging resistance elements are each disposed on a discharging path at a position that prohibits discharging of charges stored in a capacitor of the corresponding RC filter. A fault diagnosis device simultaneously turns on two discharging switches, which are apart from each other across at least two discharging switches, and determines that any of the discharging switches between the two discharging switches simultaneously turned on has a fault, when a terminal voltage of any of the battery cells that are located between the two discharging switches simultaneously turned on changes over a predetermined voltage.