A bidirectional coupler includes a first main line that transmits a first signal from a first input end thereof to a first output end thereof, a second main line that transmits a second signal from a second input end thereof to a second output end thereof, a first sub-line having a first end corresponding to the first input end and a second end corresponding to the first output end, a second sub-line having a first end corresponding to the second input end and a second end corresponding to the second output end, a detector port, a termination circuit, and a switch circuit. The first end of the first sub-line and the first end of the second sub-line are connected to each other.