A relay device includes a switch for switching a combination of an input for receiving traffic data and an output for sending the traffic data; a congestion information processing section for obtaining congestion information indicating a degree of congestion of each of a plurality of traffics from an adjacent relay device, and congestion information of each traffic in the relay device; a congestion information comparison section for finding information on a congestion level which quantitatively indicates difficulty of flowing of each traffic on a transmission route based on the congestion information obtained from the adjacent relay device and the congestion information of the relay device; a transmission scheduling adjustment section for assigning a transmission band of a bus to each traffic based on the congestion level; and a switch assignment section for shifting the switch based on a result of the assignment of the transmission band of the bus.