A communication device that controls a transmission bandwidth of TCP communication includes a means for linearly increasing the transmission bandwidth of the TCP communication during a certain period of time after a data segment whose reception has been acknowledged enters an update state without detection of congestion in the TCP communication, a means for non-linearly increasing the transmission bandwidth after the certain period of time, a means for detecting congestion using a change rate of a discarding rate or a retransmission rate in the TCP communication, a means for estimating a retransmission rate and a change rate thereof using a previous history of the transmission bandwidth and a retransmission bandwidth, and a means for estimating a discarding rate and a change rate thereof using a previous history of the transmission bandwidth and an ACK reception number.