PROBLEM TO BE SOLVED: To provide an unauthorized control restraint method capable of appropriately restraining unauthorized control due to an authorized frame circulating on a network.SOLUTION: An unauthorized control restraint method is performed by an unauthorized control restraint device 4100 connected with a bus 300 in a network system including a plurality of electronic control units that exchange a plurality of frames including a control frame to give instruction of prescribed control on a control target and a state frame including information on a state of the control target via the bus 300, i.e. a communication path. The unauthorized control restraint method includes: a reception step of sequentially receiving a plurality of frames from the communication path; and a determination step of determining whether to restrain prescribed control based on the received control frame, on the basis of a prescribed reference which is satisfied when a state of the control target in a prescribed period identified based on a set of state frames received within the prescribed period prior to reception of the control frame is a change frequent occurrence state, and is not satisfied when the state of the control target is not the change frequent occurrence state.SELECTED DRAWING: Figure 28
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