A data communication system for a computer system in which a plurality of computers are mutually connected includes: a plurality of computers each having an area to store a command to execute a data communication on the basis of it and a buffer area to store data; a switch circuit to mutually selectively connect the plurality of computers; and a transmission permitting component, connected between the switch circuit and one of the plurality of computers, for outputting a signal to permit the data transmission from such one computer to such another computer; a communication component for transmitting the data received from such one computer by outputting the transmission permission signal from the transmission permitting component to such another computer through the switch circuit; a detecting component to detect the occurrence of an abnormality regarding the data communication from such one computer to such another computer; and a communication control component to abandon the data that is subsequently received from such one computer by outputting the transmission permission signal in accordance with an output of the detecting component. When the abnormality occurs, the switch circuit releases such another computer, thereby disconnecting such one computer and such another computer. When reception commands prepared in the buffer area of such another computer are perfectly used, such another computer abandons the packet which does not include the flag to instruct the continuous reception.