PURPOSE: To control the switching of a host without an auxiliary storage device on a dam terminal side by performing the exclusive control using a communication management table indicating the communication state from each host provided on the dam terminal side.

CONSTITUTION: A communication management table 11 performing the communication management of each host 1 and a dam terminal 3 in an interface part 10 interfacing the both signals between the host 1 and the dam terminal 3 is provided to perform exclusive control against the command request from each host 1. In short, when the terminal open command from the host 1 is received, the exclusive control of the other host 1 is performed by setting the host address on communication to the response of the open command referring to the communication management table 11. Thus, the switching control of the host 1 can be performed on the dam terminal 3 side without the auxiliary storage device.