A duplexer includes: an insulation substrate having an upper surface on which a transmission filter and a reception filter are mounted, and a lower surface on which a foot pad layer electrically connected to the transmission filter and the reception filter is formed; a transmission pad provided on the upper surface and electrically connected to the transmission filter; a reception pad provided on the upper surface and electrically connected to the reception filter, a ring-shaped electrode provided on the upper surface and configured to surround the transmission pad and the reception pad; a ground foot pad included in the foot pad layer, and a via interconnection configured to electrically interconnect the ring-shaped electrode and the ground foot pad and to be provided in the ring-shaped electrode in a section along a shorter one of routes that connect the transmission pad and the reception pad to each other along the ring-shaped electrode.