PURPOSE: To improve communication efficiency by decreasing the speed of a transmission clock to be supplied from communication controller to an opposite device based on the congested state of the communication controller, and evading the deletion of a reception frame and the occurrence of re-transmission.

CONSTITUTION: The congested state of a communication control circuit 1 is detected by a congestion detection circuit 2, and an instruction to decrease the speed of the transmission clock being supplied to the opposite device (a) is issued to a transmission clock setting circuit 3. The opposite device decreases the speed of delivery data 7 to the communication control circuit 1 synchronizing with the transmission clock. Therefore, since the speed of the data receiving from the opposite device can be decreased when it is predicted that the congestion state of the communication controller occurs, it is possible to evade the deletion of a frame due to the occurrence of overrun or the re-transmission and to minimize the increment of the delay of data in the device.