To provide an analog video phone system in which real time diagnostic control and interactive diagnostic control are attained while making communication.

A system control section 5 of a maintenance center makes an area assignment revision request to a multiplex control section 10 and makes a diagnostic command transmission request to a diagnosis control section 9 and makes an area assignment revision request to the multiplex control section 10. The diagnostic control section 9 receiving the diagnostic command transmission request makes a data transfer request to the multiplex control section 10 and receives a data transfer display request to make a diagnostic response reception request to the system control section 5. The multiplex control section 10 receiving the area assignment revision request to send a proper multiplex table number and a fact of diagnostic start and end through a control line and receives the diagnostic command by the data transfer request through a data line and data transfer is displayed on the diagnostic control section 9 by the reception of diagnostic response data so as to attain interactive diagnostic control in real time.