Abstract: An interposer includes: a circuit board stack in which circuit boards are stacked; and an outer board arranged on at least one of outer side surfaces of the circuit board stack. The circuit boards are arranged include first conductive lines having first ends exposed through a first side portion of the circuit boards and second ends exposed through a second side portion opposite the first side portion of the circuit boards. The outer board includes second conductive lines having first ends exposed through a different side from the first side portion of the circuit boards and second ends exposed through a side portion located on a same side as the second side portion of the circuit boards.