Abstract: A signaling transmitting/receiving circuit between a primary multiplexer used in an ISDN and an ISDN switching system is disclosed. The signaling transmitting/receiving circuit comprises a frame arrangement unit connected between a transmission line of the primary multiplexer and a thirty-two channel PCM-highway for arranging data within a frame from thirty-two channel data to twenty-four channel data or from twenty-four channel data to thirty-two channel data; a signaling data insertion/extraction circuit having time slot registers accessible to at least one channel data and connected to the frame arrangement unit, for inserting/extracting data having a time slot width and a channel data rate according to mode assignment into/from the time slot of the PCM-highway corresponding to time slot assigning data stored in the time slot registers in response to signaling control data.