In transmitting data between a server device and a client device or between communication terminals, a data transfer rate is set in response to an available bandwidth, an application of the data, and the like not to cause waste of the network. A controlling portion detects a packet loss caused per transmission of data, and then chooses a compression mode whose data transfer rate is low when the packet loss is detected. In contrast, the controlling portion counts a time required from the data transmission (a reciprocation delay time) in such a situation that no packet loss is caused, and chooses a compression mode whose data transfer rate is high when the reciprocation delay time is sufficiently small. As the method of choosing the compression mode, for example, the method of switching a codec into another codec or changing a compression ratio being set in the codec may be employed.