A busbar module to be attached to a battery assembly having a stack of a plurality of cells, the busbar module comprising: a circuit body having a flexible printed circuit board including wiring patterns; a plurality of busbars to be connected to corresponding electrodes of the plurality of the cells; and a holder for holding and housing the circuit body and holding the busbars and being extendable in a stacking direction of the plurality of the cells so that the busbars are movable in the stacking direction, the circuit body having: a plurality of conductor layers (203a, 203b) and a plurality of protective layers to form a multiple-layered structure of the wiring patterns; a band-shaped main strip to be located to extend in the stacking direction; and a band-shaped branch strip branched from the main strip; wherein the main strip and the branch strip are a flexible printed circuit board, the branch strip having: a bent portion extending in the stacking direction and having a bent shape around an axis, wherein the axis crosses the stacking direction and extends parallel with the width direction of the main stripe; and a connection portion disposed closer to an end of the branch strip than the bent portion and connected to the corresponding busbar, the bent portion having a thin-layer portion having a shape formed by removing, from the flexible circuit board, a part of the protective layers corresponding to a part of the conductor layers (203a, 203b) without being used as the wiring pattern in the branch strip wherein a width of a part of the wiring pattern passing through the thin-layer portion is smaller than that of other parts of the wiring pattern passing through a portion of the branch strip other than the thin-layer portion. a circuit body having a flexible printed circuit board including wiring patterns; a plurality of busbars to be connected to corresponding electrodes of the plurality of the cells; and a holder for holding and housing the circuit body and holding the busbars and being extendable in a stacking direction of the plurality of the cells so that the busbars are movable in the stacking direction, the circuit body having: a plurality of conductor layers (203a, 203b) and a plurality of protective layers to form a multiple-layered structure of the wiring patterns; a band-shaped main strip to be located to extend in the stacking direction; and a band-shaped branch strip branched from the main strip; wherein the main strip and the branch strip are a flexible printed circuit board, the branch strip having: a bent portion extending in the stacking direction and having a bent shape around an axis, wherein the axis crosses the stacking direction and extends parallel with the width direction of the main stripe; and a connection portion disposed closer to an end of the branch strip than the bent portion and connected to the corresponding busbar, the bent portion having a thin-layer portion having a shape formed by removing, from the flexible circuit board, a part of the protective layers corresponding to a part of the conductor layers (203a, 203b) without being used as the wiring pattern in the branch strip wherein a width of a part of the wiring pattern passing through the thin-layer portion is smaller than that of other parts of the wiring pattern passing through a portion of the branch strip other than the thin-layer portion. a plurality of conductor layers (203a, 203b) and a plurality of protective layers to form a multiple-layered structure of the wiring patterns; a band-shaped main strip to be located to extend in the stacking direction; and a band-shaped branch strip branched from the main strip; wherein the main strip and the branch strip are a flexible printed circuit board, the branch strip having: a bent portion extending in the stacking direction and having a bent shape around an axis, wherein the axis crosses the stacking direction and extends parallel with the width direction of the main stripe; and a connection portion disposed closer to an end of the branch strip than the bent portion and connected to the corresponding busbar, the bent portion having a thin-layer portion having a shape formed by removing, from the flexible circuit board, a part of the protective layers corresponding to a part of the conductor layers (203a, 203b) without being used as the wiring pattern in the branch strip wherein a width of a part of the wiring pattern passing through the thin-layer portion is smaller than that of other parts of the wiring pattern passing through a portion of the branch strip other than the thin-layer portion. a bent portion extending in the stacking direction and having a bent shape around an axis, wherein the axis crosses the stacking direction and extends parallel with the width direction of the main stripe; and a connection portion disposed closer to an end of the branch strip than the bent portion and connected to the corresponding busbar, the bent portion having a thin-layer portion having a shape formed by removing, from the flexible circuit board, a part of the protective layers corresponding to a part of the conductor layers (203a, 203b) without being used as the wiring pattern in the branch strip wherein a width of a part of the wiring pattern passing through the thin-layer portion is smaller than that of other parts of the wiring pattern passing through a portion of the branch strip other than the thin-layer portion.