To provide a relay device and its QoS control method which effectively uses the band of an Ethernet line.

In a relay device placed on each location of a wide-area LAN service which multiplexes a plurality of logical lines into a single physical line having a predetermined transmission band to be transmitted to another relay device, data flowing from a plurality of logical lines are associated with a physical line to be grouped, and a QoS control is executed to a single data flow grouped. The data flow is distributed to a plurality of micro-flows based on header information of the data flow grouped, and a priority of a transmission and a transmission band are set to each micro-flow. Policing is executed by the transmission band set, and scheduling of transmission processing is executed to a plurality of micro-flows to which policing is performed based on a priority assigned thereto.