To provide a secondary battery circuit in which a deteriorated unit cell can be safely cut-off by preventing expansion of damage to an internal short circuit of the unit cell and not applying a high voltage to a fuse when the fuse is blown.

A secondary battery unit includes: unit cells; a battery pack constituted by connecting a plurality of unit cells E1, E2, ... En in parallel; unit fuses F1, F2, ... Fn serially-connected to the respective unit cells in the battery pack; and a diode D having an anode connected to a negative electrode side of the battery pack and a cathode connected to a positive electrode side of the battery pack. The secondary battery unit is constituted by connecting the battery packs in series.