PURPOSE: To prevent the interruption of a program except for disconnecting instructions and to reduce the load of a program processing by separately detecting the entire 24-bit continuation states of an R line and a 1 line and the times of the 24-bit continuation and the disconnecting instruction.

CONSTITUTION: A communication controller 200 for connecting line by an X21 interface and performing transmission/reception is provided with line controllers 500-50n. Then, the state of the R and 1 lines from a line terminating equipment 600 is detected, an interruption signal for indicating the same 24-bit state and the interruption signal for indicating only the disconnecting instruction state are outputted, and also the suppression of the interruption by the interruption signal in each state is set and released by the control program. When the state of the lines turns to the state in which the monitoring is unnecessary except for the disconnecting instructions, the detection and the interruption of the same 24-bit state are suppressed by a control program, and only by detecting disconnecting instruction, 24-bit identification and the disconnection instruction can be separately detected.