A host computer in a communication system measures a round trip time (RTT) returned from a destination terminal when a connection is established on a virtual communication path on a network interface card (NIC) for each connection. When the RTT is larger than a predetermined threshold, the host computer starts storing transmission data in a retransfer buffer in an OS of the host computer, and transfers the transmission data to the NIC. When the RTT is smaller than the predetermined threshold, the host computer starts transferring the transmission data to the NIC and stores the transferred transmission data in a retransfer buffer in the NIC.