Security is maintained in a data communication system while minimizing the frequency with which communication is carried out. A data transmitting server and a relay server communicate based upon https utilizing port 443 of a firewall. When query data is transmitted from the data transmitting server to the relay server, the relay server determines whether there is a data request from a mobile telephone within a fixed period of time. If there is a data request from the mobile telephone within the fixed period of time, the requested data is transmitted from the data transmitting server to the relay server in response to this data request. This requested data is transferred from the relay server to the mobile telephone.