Abstract: A private automatic branch exchange includes a four-wire type extension modular jack for connection to a four-wire type analog key telephone set as an extension terminal device and for connection to both a two-wire type digital key telephone set and a standard two-wire telephone set as extension terminal devices. Event determining circuitry determines whether or not an event has been received and the kind of extension terminal device concerning the event and outputs a control signal accordingly. Analog/digital key telephone set interface circuitry makes a data signal exchange with an analog key telephone set or a digital key telephone set connected to the extension modular jack. Standard telephone set interface circuitry makes an analog signal exchange with an analog key telephone set or a standard telephone set connected to the extension modular jack.