Disclosed is a waveguide adapter able to generate a circularly polarized wave. The waveguide adapter to be coupled with a horn antenna realizes a polarized wave conversion function for converting a linearly polarized wave signal into a circularly polarized wave signal, or vice versa, and an adapter function for converting a waveguide signal into an external transmission line signal, resulting in a simplified configuration and small size of a communication system using a circularly polarized wave signal. The waveguide adaptor includes a probe to transmit a linearly polarized wave signal from an external transmission line to a waveguide transmission line, a polarized wave conversion line reflector located in the rear of the probe to convert a vertically polarized wave into a horizontally polarized wave, and a back-short member to forwardly transmit a rearward signal. The waveguide adapter is applicable to communication systems using circularly polarized wave signals.