PURPOSE: To realize the control of an input/output buffer with sufficiently high efficiency having small buffer capacity, by sorting temporatily the types of buffer requests into a buffer and a holding buffer, and making both buffers share a region in the same buffer.
CONSTITUTION: A master station control program 8 contains a buffer control table 9, a holding buffer status table 10, an input/output buffer 11, etc. The information showing a temporary buffer or a holding buffer and a state display showing that the unit is not used, is in use and is being held respectively are registered to the table 9 in response to each unit in a buffer region. For a buffer using request, the units of a buffer 11 are allotted successively from an end of the buffer 11 for the temporary buffer and from the other end of the buffer 11 for the holding buffer respectively on the basis of the table 9. As a result, the access speed is increased for the data which is used repetitively. In addition, the using efficiency of buffers can be increased owing to the sharing a region in the same buffer. This assures a sufficient effect with small buffer capacity.