out after arrival of data after execution of the data transfer control the data transfer is finished step 104 in the transfer control steps 101 201 to 210 of data for the buffer capacity or less a request for data having a capacity of the buffer 5 or less is indicated to the optical channel 1 once the request for data having a capacity of the buffer 5 or less is first indicated in step 101 the process does not stop and wait for the data as provided in the prior art but proceeds to the data transfer control before arrival of data at the next step 102 in the data transfer control steps 102 301 to 310 before arrival of data the optical communication control unit 4 monitors a logical empty state of the buffer and sends a further data request to the optical channel 1 in the monitoring process of the logical empty state of the buffer the data transfer from the buffer 5 to the channel control unit 7 by the physical hardware control is not performed before arrival of data and accordingly the empty state of the buffer 5 is monitored on the basis of an average data transfer speed of the hardware control for each transfer control request for data corresponding to the buffer capacity or less at step 101 to indicate the data request thereafter when the first data corresponding to the buffer capacity or less requested at step 101 is received the process proceeds to the data transfer control after arrival of data.