PURPOSE: To automatically release the connection, in the case a communication error is generated on the connection between a server and a client by a TCP/IP.

CONSTITUTION: In a TCP layer of a client 1, in the case a communication error is generated at the time of data communication by a connection, a data transmission/reception processing part 11 detects its error, a connection control part 13 closes the connection, and a connection establishment request part 12 outputs a connection establishment request until the connection is established. On a server 2 side, a connection establishment passive part 22 receives its establishment request and establishes the connection, and by the normal connection concerned, data is subjected to communication by the data transmission/reception processing parts 11, 21.