To provide a system/method for controlling data communication capable of supplying equipment information to a control tool on an interruption side even in an exclusive data communication state.

In data communication environment where a plurality of control tools A and B exclusively make data communication possible to one controller (2, 11) to connect other controllers (1, 4), the tools A and B are respectively provided with at least data communication means (7 and 8) for processing communication between with the one controller (2 and 11) and equipment information control means (8 and 10) which retains the equipment information of the one controller (2 and 11) on starting communication between with the one controller (2 and 11) in an equipment information control part 6 disposed in the other controllers (1 and 4) and which allows the other control tools B and A to refer to the equipment information retained in the part 6 of the controllers (1 and 4) disposing the other tools B and A when the one controller (2 and 11) is in communication between with the other tools B and A in the case of trying to start communication between with the one controller (2 and 11).