PURPOSE: To control a receiving buffer with neither omission of received data nor the influence caused by a communication means, etc., before reception of data.

CONSTITUTION: A receiving buffer margin 31 is secured in a receiving buffer memory 30 as a margin area for an actual data full state. The value of the margin 31 is set by a receiving buffer controller 20. When the margin 31 is set, a transmission time counting part 21 of the controller 20 counts the time needed for reception of data from the data transmitter side. Then, a data quantity calculating part 22 calculates the quantity of data transmitted within the counted transmission time based on this time. And, a receiving buffer margin setting part 23 sets the value of the margin 31 in order to be able to store the quantity of data calculated by the part 22.