A communication terminal (1) on a transmission side comprises means (1-10) for detecting congestion, means (1-7) for detecting a sign of congestion with reference to an ideal congestion window size and a current congestion window size, means for estimating a ratio of priority traffic with respect to traffic within a network, and means (1-8) for dynamically changing a threshold value for detection of the sign of congestion in response to the estimated ratio. When the congestion or the sign of congestion is detected, a congestion window size is changed into the idea congestion window size.