An interposer includes a stacked body including insulating base material layers that are stacked on one another, first and second electrodes, a conductor pattern, and an interlayer connection conductor. The stacked body includes a first mounting surface including a first electrode, and a second mounting surface facing the first mounting surface and including a second electrode. The first electrode is electrically connected to the second electrode through the conductor pattern and the interlayer connection conductor. A length of an electrical path including conductor patterns connecting the first electrode and the second electrode is larger than a total length of the interlayer connection conductor in a stacking direction.