An in-vehicle relay device for a vehicle is provided. The in-vehicle relay device includes a first communication circuit that transmits and receives first data to and from a first in-vehicle equipment in the vehicle. A second communication circuit transmits and receives second data to and from a second in-vehicle equipment in the vehicle. A control circuit controls relay of third data among the first communication circuit and the second communication circuit. In a case where reception data received by the control circuit from the first communication circuit or the second communication circuit is unauthorized, the control circuit stops relaying the third data in accordance with a traveling state of the vehicle.