A relay device may include a plurality of communication circuits and a state control portion. The relay device may relay the data among a plurality of buses, each of which connecting one or more nodes. The plurality of communication circuits may perform transmission and reception of the data, and transition among a plurality of states including a transmission enabled state and a transmission disabled state. The plurality of communication circuits may be connected to the plurality of buses in one to one manner. The state control portion may cause a specific communication circuit to transition from the transmission enabled state to the transmission disabled state when a relay transition time has elapsed from an occurrence of predetermined event. The specific communication circuit may represent at least one of the plurality of communication circuits to which at least one of the plurality of buses connecting the specific node is connected.