1. A communication apparatus comprising: a receiving antenna that receives analog signals; an A/D converter that digitizes the received analog signal; a noise eliminator, connected to the last stage of the A/D converter, that performs noise reduction processing to cancel an input digitized signal; a code decoder, connected to the last stage of the noise eliminator, that decodes the digitized signal that has been subjected to noise reduction processing; a desired-signal detector that limits the noise reduction processing by the noise eliminator when a digitized signal corresponding to a known form of modulation of a desired signal is received by the code decoder; a code interpreter, connected to the last stage of the code decoder, that interprets a decoded signal obtained from decoding by the code decoder; and a controller that disables limitation of the noise reduction processing when a decoded signal not corresponding to a known form of encoding of a desired signal is received by the code interpreter.