The present invention is directed to a stereo/monaural selecting circuit having a jack for supplying therefrom for reproduction a stereophonic sound signal output or a monaural sound signal output to apparatus connected thereto, or for receiving for recording a stereophonic sound signal input or a monaural sound signal input from apparatus connected thereto. The jack can be used with either a binaural plug or a monaural plug inserted thereinto, which are attached to stereophonic apparatus or monaural apparatus, respectively, and the circuit selects automatically either a stereophonic operating state or a monaural operating state in response to the type of plug inserted into the jack. In the case where the circuit contains a speaker, the circuit supplies the monaural sound signal output to the speaker when no plug is inserted into the jack.