PURPOSE: To guarantee an optional operating time of a terminal device by one kind of command by adding a data representing the guaranteeing value of the operating time of terminal device into a transfer data transferred from a central processing device.

CONSTITUTION: A communication controller 2 decides whether a received data is a control data (a) or not at each reception of the data reserved by the central processing unit 1 and transfers data A, B other than the control data (a) all to a terminal device 3. When the control data (a) is detected, the device 2 discriminates that data b1, b2 transferred next represent time values tb1, tb2 for guaranteeing the operating time of the terminal device 3 and dose not transmit data to the terminal device 3 until the time values are elapsed to attain non- communication state. After the elapse of guaranteeing time, the communication controller 2 continues the data transmission to the terminal device 3 while discriminating whether the received data is the control data (a) or not again.