The present disclosure relates to an interposer (120), which is a circuit board that has a multilayer structure and that establishes a connection between layers using a via conductor. The interposer (120) includes first and second transmission lines that are connected in series and a first stub and a second stub that are respectively connected to the first transmission line and the second transmission line. The first and second stubs are formed by wiring lines provided in respective different layers, and a second transmission line (123), which connects the first stub to the second stub, includes a via conductor and a wiring line provided in the layer where the second stub (124) is formed.