A communication system is provided with a transmission terminal and a reception terminal that communicate over a network, and the transmission terminal includes: an encoded packet generation unit which encodes a predetermined number of packets and transmits the encoded packets to the reception terminal over the network; a network environment measurement unit which measures the state of the network; and a parameter calculation unit which determines the predetermined number of packets by using a characteristics to be guaranteed in regard to delay time required for a packet transmitted from the transmission terminal to arrive at the reception terminal and the state of the network. In the communication system in which the transmission terminal and the reception terminal communicate via the network on which a packet loss may occur, a characteristic regarding a packet delay time specified by a user is guaranteed.