A vehicular circuit body includes a trunk line that extends in at least a front-and-rear direction of a vehicle, a plurality of control boxes that are provided on the trunk line, and a branch line that connects the control box to an accessory. The trunk line and the branch line each includes a power source line having a predetermined current capacity and a communication line having a predetermined communication capacity. The control box includes a branch line connection portion to which the branch line is connected, and a branch line control unit that distributes power from the trunk line to the branch line by controlling the branch line connection portion according to a control program. The control program can be externally changed based on the accessory connected to the branch line.