PURPOSE: To share one antenna at plural arbitrary frequency bands by providing a semi-optical demutiplexing filter in the resonator of the spiral antenna.

CONSTITUTION: A radio wave to be radiated from the antenna is composed of a radio wave component to be radiated from radiators 21 and 22 to a free space directly and a radio wave component to be radiated from the radiators 21 and 22 to the side of a resonator 1 and to be radiated to the free space after being reflected on the bottom of the resonator 1 or on a semi-optical demultiplexing filter 5. The radio component to be temporarily radiated to the side of the resonator 1 by the radio wave of a frequency fA is transmitted through the demultiplexing filter 5, reflected on the bottom of the resonator 1, transmitted through the demultiplexing filter 5 again and radiated to the free space. The phases of radio waves in the two components having this frequency fA are made same phase in the free space since a distance A from the radiators 21 and 22 to the bottom of the resonator 1 is set on the condition of A=λA/4. On the other hand, even in the case of the radio wave having a frequency fB, it is made similar as well.