PURPOSE: To prevent the interruption of exchanging operation by setting a counter judging whether a control instruction is valid or not and removing the timing of instruction cancellation.

CONSTITUTION: Instruction control part CNC 304 for the line concentrator which equips counter CHN 305 with the finite bit length judges whether the responce of a control instruction is valid or not by means of comparison of the number with that of CHN 305. When an instruction to line concentrator LC 306 can not be executed temporarily and an instruction to other LC or instruction control part SPC 303 for the central exchange can be executed, buffer SPCQ 302 for control instructions can not execute exchanging operation. For making possible the exchanging operation, an instruction in SPCQ 302 is read and is loaded again in SPCQ 302; at the same time, advancing operation of CHN 305 is executed. According to the method above mentioned, the guarantee of a card timing for instruction reloading is not required and the interruption of exchanging operation is prevented.