A busbar module comprising: a plurality of busbars arranged along a first direction (X) and connected to battery cells , wherein a second direction (Y) is a direction orthogonal to the first direction (X) in a plan view of the busbar module and also a width direction of the busbar-module ; and a plate-like circuit body having a plurality of detection lines to be connected to the busbars and having flexibility, wherein the circuit body includes a main line portion in which the plurality of the detection lines is routed and a plurality of branch portions branching from the main line portion , wherein in each of the branch portions , one of voltage detection lines among the detection lines to be connected to one of the busbars is routed, the branch portion includes a base having L-shape, connected to the main line portion and protruding from the main line portion in the second direction (Y), a distal end located at the distal end of the branch portion and connectable to one of the busbars , and a stretchable portion located between the base and the distal end , and stretchable along the first direction (X) by having a plurality of slits and cuts formed in the branch portion in the second direction (Y) wherein, the stretchable portion has the plurality of the slits extending in the second direction (Y) and a plurality of cuts extending in the second direction (Y), the plurality of the slits includes a plurality of first slits extending toward one side in the second direction (Y) and a plurality of second slits extending toward another side in the second direction (Y), the first slits and the second slits face each other in the second direction (Y), and the cuts are each disposed between a pair of one of the first slits and one of the second slits and adjacent another pair of one of the first slits and one of the second slits , and the voltage detection line of the stretchable portion is routed along the slits and the cuts , and is housed in the branch portion . a plurality of busbars arranged along a first direction (X) and connected to battery cells , wherein a second direction (Y) is a direction orthogonal to the first direction (X) in a plan view of the busbar module and also a width direction of the busbar-module ; and a plate-like circuit body having a plurality of detection lines to be connected to the busbars and having flexibility, wherein the circuit body includes a main line portion in which the plurality of the detection lines is routed and a plurality of branch portions branching from the main line portion , wherein in each of the branch portions , one of voltage detection lines among the detection lines to be connected to one of the busbars is routed, the branch portion includes a base having L-shape, connected to the main line portion and protruding from the main line portion in the second direction (Y), a distal end located at the distal end of the branch portion and connectable to one of the busbars , and a stretchable portion located between the base and the distal end , and stretchable along the first direction (X) by having a plurality of slits and cuts formed in the branch portion in the second direction (Y) wherein, the stretchable portion has the plurality of the slits extending in the second direction (Y) and a plurality of cuts extending in the second direction (Y), the plurality of the slits includes a plurality of first slits extending toward one side in the second direction (Y) and a plurality of second slits extending toward another side in the second direction (Y), the first slits and the second slits face each other in the second direction (Y), and the cuts are each disposed between a pair of one of the first slits and one of the second slits and adjacent another pair of one of the first slits and one of the second slits , and the voltage detection line of the stretchable portion is routed along the slits and the cuts , and is housed in the branch portion . <EOS>