PURPOSE: To attain the transfer of a file through the same circuit processing circuit as that used for transmission of the screen data, by using a switching circuit which performs changeover between the screen data and the file transfer data and a file transfer control circuit which controls the file type for transfer.

CONSTITUTION: A file transfer control circuit 9 adds a file transfer switching code 15 and a file transfer style code 16 at the head of the data read out to a file transfer buffer 10. Then this data is sent to a host computer via a switching circuit 8 and a circuit processing circuit 1. The maximum data length sent to a communication line 7 is generally larger than the section of the data read out of a file device 13. Thus the data read out of the device 13 is stored and buffed until it has the same length as the data length that can be sent out. Then this data is sent to a circuit at that time point when it can be sent out. Here the device 13 has plural pieces of section data length and therefore the section code is put among those data lengths. This decreases the transmitting/receiving frequency of the acknowledge to the circuit in a data transmission mode. As a result, the circuit efficiency is improved.