PURPOSE: To improve the utilizing efficiency of a line by detecting whether or not a signal line is in use, suppressing the switching operation when in use, detecting furthermore the switching state of a line, and informing it to a high- order device to discriminate the operation disable line.

CONSTITUTION: One signal line of data signal lines 20, 21 is connected to a signal line 20 and the other signal line is connected to a switching device state informing section 2. The informing section 2 reports the disconnecting state or the switching operation to the high-order device to which the signal line is connected. When a switch control section 4 receives a switching signal from an external part, the section 4 outputs a connection duration signal to the signal lines 12, 13 and a switching circuit state detection section 1 outputs a connection destination signal 14. A signal line operating state detection section 3 detects the operating state of the signal line during connection and reports it to the control section 4. In case of non-operating state, the control section 4 switches the signal line. Moreover, the detection section 1 informs it to the informing section 2 that the switching circuit is in operation through a signal line 16.