There is disclosed a data circuit-terminating equipment (DCE) which connects a start-stop synchronous data terminal equipment (DTE) which is not synchronized with a PCM transmission line having various speeds. Further the DCE can satisfy recommendations of the V25 bis of CCITT. The DCE includes a start-stop synchronizing circuit to deliver a sampled request-to-send signal RS, a sampled send data and to transmit a clear-to-send signal CS to the DTE, a PLL obtaining a clock from the line, a timing generator generating timings for circuits, a mapping circuit mapping to make the sampled send data match into the line speed, a sending register converting transmission speed of the mapping circuit output to send to the line at the instructed period, a receiving register receiving data from the line to deliver data with the required speed during the required period for the DTE, and a demapping circuit receiving the receiving register output to demap and send to the DTE. In the DCE satisfying the recommendations, the demapping circuit ANDs a clear-to-send signal and bits indicating the synchronized state to send the clear-to-send signal, and a short line shorting the clear-to-send signal and a carrier detect signal is employed.