A first loss determination section 13 of a first communication device 10 determines that a data unit is lost if an acknowledgment response for the transmitted data unit from a second communication device 30 is not received within a predetermined time and retransmits the data unit when the determination is made. When a determination section 16 determines that a spurious timeout has occurred, a window control section 17 sets a target value for decreasing the size of a congestion window and then gradually decreases the size of the congestion window to the target value when the acknowledgment response is received. Therefore, in a case of executing a response algorithm for changing a slow start threshold and the congestion window, if a segment loss occurs subsequent to the spurious timeout, an efficient loss recovery is achieved and the throughput is improved.