PURPOSE: To transfer data at high speed between asynchronous devices different in processing speed and to transfer data at high speed between devices different in data width.

CONSTITUTION: An external device 4 transmitting data executes write access only when a free area exists in the buffer memory area of a memory 2, which is designated by the area designation part of a buffer memory management part 9, by the control of the buffer memory management part 9 and an access control, part 10. An external device 5 receiving data executes read access to the buffer memory area only when data is stored in the buffer memory area by the control of the buffer memory management part 9 and an access control part 11. Then, the external devices 4 and 5 different in processing speed asynchronously transfer data.