A memory bandwidth control device for improving efficiency of data transfer between an external device and a memory. A memory is used for temporarily storing and outputting data to be communicated with external devices being connected via input/output ports, and an I/O buffer dynamically assigns memory bandwidth to external devices requesting data transfer when receiving the data transfer requests from the external devices to the memory. When compared to assigning fixed memory bandwidth, efficiency of data transfer to the memory can be significantly improved without wasting memory bandwidth.