An in-vehicle relay device prevents an anomaly of a control command exchanged over networks having different transmittable data sizes in a frame. The in-vehicle relay device relays communication between multiple control devices in a vehicle over the networks to which the control devices are connected. The in-vehicle relay device receives control data from a first control network. The control data includes, in a frame, a plurality of control commands to be executed by at least one of the control devices. The in-vehicle relay device determines, as a first determination, whether types of the control commands included in the frame form a first combination that is preset as a combination of control commands that are executable simultaneously. The in-vehicle relay device thereafter determines, as a second determination, whether the control data is anomalous by using the result of the first determination, and outputs the result of the second determination.