PURPOSE: To prevent the stay of transmission data in a communication control processor by permitting a network program to instruct the notice of an instruction signal for the network program which hardware controlling a data link layer protocol executes to execute suppression and release.

CONSTITUTION: Interruption occurring from hardware 13 controlling the data link layer comes to be only the notice of a recognition signal showing the completion of data transmission in a buffer lacking state, and the use of a buffer in the network program 14 by the reception of new data is eliminated. Then, the completion of data transmission is communicated at any time. Thus, a next data transmission request operation and the buffer abolishment of data whose transmission is completed can be executed and the cancel of the buffer lacking state in the network program 14 is accelerated. Thus, a buffer lacking state continuous period can be shortened and efficient data communication accompanied by it can be realized.