PURPOSE: To control a call between a DTE(data terminal equipment) and a DCE (line terminating device with call control function) by transmitting transmission data of the DTE, which does not pass a ciphering circuit, and transmitting reception data, which does not pass a deciphering circuit, to the DTE at the time of settling the call.

CONSTITUTION: When call control information is exchanged between a DTE 1 and a DCE 2, a data set ready signal is kept turned off. Consequently, call control information sent from the DCE 2 is supplied to a terminal I2 of a selecting circuit 32 by a signal line 303 and is supplied to the DCE 2 from a terminal Qby a signal line 306 because of turning-off of an input terminal I3. Call control information sent from the DCE 2 is supplied to a terminal I2 of a selecting circuit 33 by a signal line 307 and is supplied to the DTE 1 from a terminal Q by a signal line 302 because of turning-off of an input terminal I3 of the selecting circuit 33. Thus, unciphered call control information is exchanged between the DTE 1 and the DCE 2 to settle the call.