An unauthorized control suppression method in a network system (10; 11; 12), the network system (10; 11; 12) including a plurality of electronic control units (200a - 200d; 3200e, 3200f) that exchange, via a communication channel, a plurality of frames comprising a plurality of data frames, the method comprising: a reception step of sequentially receiving a plurality of data frames from the communication channel; characterized by said plurality of data frames received in the reception step including a control frame that instructs predetermined control to an object of control; and a determining step of determining whether or not the predetermined control, instructed by said control frame received in the reception step, should be suppressed, based on a set of frames of said plurality of data frames received in the reception step, wherein said set of frames is received within a predetermined period preceding a time of reception of the control frame received in the reception step, wherein the plurality of data frames received in the reception step further include a state frame including information relating to a state of the object of control, wherein, in the determining step, whether or not the predetermined control based on the control frame received in the reception step should be suppressed is determined based on whether or not the state of the object of control within the predetermined period preceding the time of reception of said control frame, satisfies a predetermined reference, wherein the state of the object of control is identified based on a set of the state frames received in the reception step within the predetermined period preceding the time of reception of said control frame, wherein the predetermined reference is satisfied in a case where an inconsistency continuation time of an inconsistency for the state of the object of control in the predetermined period exceeds a predetermined time, and the predetermined reference is not satisfied in a case where the inconsistency continuation time does not exceed the predetermined time, wherein the inconsistency continuation time is a time over which an inconsistency in a change of state continues; and wherein, in the determining step, determination is made that the predetermined control should be suppressed in a case where the predetermined reference is satisfied. a reception step of sequentially receiving a plurality of data frames from the communication channel; characterized by said plurality of data frames received in the reception step including a control frame that instructs predetermined control to an object of control; and a determining step of determining whether or not the predetermined control, instructed by said control frame received in the reception step, should be suppressed, based on a set of frames of said plurality of data frames received in the reception step, wherein said set of frames is received within a predetermined period preceding a time of reception of the control frame received in the reception step, wherein the plurality of data frames received in the reception step further include a state frame including information relating to a state of the object of control, wherein, in the determining step, whether or not the predetermined control based on the control frame received in the reception step should be suppressed is determined based on whether or not the state of the object of control within the predetermined period preceding the time of reception of said control frame, satisfies a predetermined reference, wherein the state of the object of control is identified based on a set of the state frames received in the reception step within the predetermined period preceding the time of reception of said control frame, wherein the predetermined reference is satisfied in a case where an inconsistency continuation time of an inconsistency for the state of the object of control in the predetermined period exceeds a predetermined time, and the predetermined reference is not satisfied in a case where the inconsistency continuation time does not exceed the predetermined time, wherein the inconsistency continuation time is a time over which an inconsistency in a change of state continues; and wherein, in the determining step, determination is made that the predetermined control should be suppressed in a case where the predetermined reference is satisfied.