Improving the audio QoS of a video call service in a weak signal environment is accomplished. To do this, even if data is damaged in a weak signal environment during a video call using a mobile station in the weak signal environment, an audio call is preferentially processed such that a minimum audio QoS can be secured. If a transmitting mobile station enters into a weak signal environment while transmitting audio data and video data together during a video call, the transmitting mobile station transmits only audio data, and in response to this, a receiving mobile station processes the audio data transmitted during a set time interval and continuously reproduces the audio data. Audio interruption can be reduced, thereby securing a minimum audio QoS.