In order to avoid deterioration in sound quality caused by band shortage between an HNB-GW and an HNB, in a femto system 100, an HNB-GW 140 is connected to an HNB 120 via a network 130, and relays at least one of transmission and reception of voice data between an MGW 170 and the HNB 120. A narrow band determination unit 150 in the HNB-GW 140 determines whether the band of the network 130 is a narrow band which is equal to or lower than a predetermined threshold. When it is determined that the band is a narrow band by the narrow band determination unit 150, a conversion unit 160 converts the voice data to decrease a bit rate of the voice data transferred to the HNB 120 from the MGW 170.