The objective of the invention is to provide a circuit body part, an electric connection box, and a wire harness which can secure proper assembling property. The circuit body part (1) applied to the electric connection box (100) of the wire harness (WH) is characterized in that the circuit body part is provided with a component housing (2) having a communication hole (20) communicating with an inside and an outside, and having a box shape; and a bus bar (3) stored in the component housing (2) and having a terminal part (3b) exposed to the outside via the communication hole (20); the componenthousing (2) is formed in a cuboid shape which matches the shape of a mounting space part (109) to which a relay (R) formed in a cuboid shape can be fitted, and can be fitted in the mounting space part(109); and the bus bar (3) forms a circuit via the terminal part (3b) in a state that the component housing (2) is fitted in the mounting space part (109).