A phase shifter includes a signal conductor constituting a transmission line for a signal transmitted through an antenna element, a dielectric plate including a dielectric material disposed to face to the signal conductor, and a mobile mechanism for moving the dielectric plate. A facing area between the signal conductor and the dielectric plate is changed by a movement of the dielectric plate, to change a phase of the signal transmitted through the signal conductor. The dielectric plate includes a transformer unit for impedance matching between an overlapped portion in which the signal conductor faces to the dielectric plate and a non-overlapped portion in which the signal conductor does not face to the dielectric plate. The signal conductor includes an input-side signal conductor which extends from the non-overlapped portion to the overlapped portion via the transformer unit, and an output-side signal conductor which is electrically connected to an end of the input-side signal conductor on a side of the overlapped portion and extends from the overlapped portion to the non-overlapped portion without any intervention of the transformer unit.