In a voice communication system 1, a gateway server 4 receives IP packets from the Internet, converts PCM voice data in the IP packets into AMR encoded voice data frames, and transmits to a mobile terminal 7. During the propagation to the gateway server 4, there is a possibility of loss of IP packets and crucial bit error in IP packets. In that case, the gateway server 4 puts "No data" data on frames as voice encoded data for the IP packets in question and sends it to the mobile terminal 7. The "No data" data is a target of concealment.