An Internet telephony system is capable of not only restraining a delay of transfer of a voice packet but also transferring a data packet at a high efficiency by fragmenting the data packet and thus routing the data packet only when the voice packet flows. The Internet telephone system has an Internet network for transferring voice information as the voice packet which is transmitted from a voice communications terminal incorporating a voice communications function, and transferring data as the data packet which are transmitted from a data communications terminal incorporating a data communications function. This system includes a first detecting unit for detecting a transition of a call-out state of the voice communications terminal, a packet assembling unit for assembling, when the first detecting unit detects the transition of the call-out state of the voice communications terminal, a control packet containing indication information for changing, to a predetermined limit value, a maximum length of each of the data packets transferred via the Internet network, and a first routing unit for routing, when receiving the control packet containing the indication information, each of the data packets and the voice packet to the Internet network while restricting the maximum length of the data packet to the predetermined limit value.