A self-diagnosis circuit is coupled to a signal transmission path between a high-frequency signal input part and an output part and is configured to diagnose a high-frequency circuit using a signal that propagates along the signal transmission path. The self-diagnosis circuit includes a detector configured to detect the high-frequency signal propagating along the signal transmission path from the output part toward the input part; and a diagnosis unit that diagnoses for an abnormality in the follower stage of the high-frequency circuit disposed between a coupling part of the self-diagnosis circuit with respect to the signal transmission path and the output part, in accordance with the detection result of the detector.