In a jack circuit, in which either positive pole side terminals or negative pole side terminals of connection terminals for a microphone and of connection terminals of an output stage amplifier for outputting to a speaker form a common terminal, the output stage amplifier is an amplifier which jointly operates with a center amplifier performing virtual grounding operation and which operating source voltage is lower than the operating voltage of the microphone, the common terminal is connected to the center amplifier and the other of either the positive pole side terminal or the negative pole side terminal for the microphone is connected to a voltage line which is higher by a level to ensure the operating voltage thereof.