A failure-section determining device includes a loss determining unit that calculates a receiving-time interval using a receiving time of data and a receiving time of immediately previous data, determines that a loss occurs when the receiving-time interval is a predetermined threshold or higher, and calculates the number of losses using the receiving-time interval and a data sending-time interval; and a determining unit that calculates the number of losses occurring in a first section between a sending device and a relay device, using a serial number of the received packet and a serial number of an immediately previous packet, and calculates the number of losses occurring in a second section between the relay device and a monitoring device, using the number of losses calculated by the loss determining unit and the number of losses occurring in the first section.