PURPOSE: To attain smooth conversation by informing of a user of a 1st voice of restarting conversation immediately after interrupting the conversation once without delay.

CONSTITUTION: A self-station sound level detecting circuit 14 detects a self- station sound level inputted from a microphone 2 and an opposite station sound level detecting circuit 16 detects the sound level of sound data transmitted from an opposite station. A sound and silence judging part 15 judges whether each sound data are sound or silence, and when both sound levels are silent for a fixed time or more, judges the interruption of conversation. When the conversation is started from either one of the self-station and the opposite station, the judging part 15 outputs a signal indicating the restart to a sound delay control part 17. When the control part 17 receives a signal indicating the restart of the conversation from the self-station first, the storage capacity of a transmitting side RAM 10 is changed only immediately after the reception and the delay of transmitting sound is minimized. When the opposite station restarts the conversation first the storage capacity of a receiving side RAM 10' is changed and the delay of receiving sound is minimized.