PURPOSE: To enable another additional service to start up while one of additional service is being started up by displaying the additional service that can be started up at present.

CONSTITUTION: An additional service controller 13 judges the working status of a channel, the state of a call trying to start up service, the state of the call with respect to the above call, and whether or not it is possible to start up the additional service by always managing a feature state based on a control channel signal from an ISDN line, and sends a judged result to the display devices of input/output devices 14a-14c. The display means of the input/output devices 14a-14c display the judged result. Also, when the mode of the additional service is started up from the input means of the input/ output devices 14a-14c, the information is inputted to the additional service controller 13. The additional service controller 13 immediately judges the result, and displays the result on the display means of the input/output devices 14a-14c, Thereby, it is possible to easily perform the start-up of the additional service.