A fraud detection electronic control unit is connected to an electronic control unit through an in-vehicle network system. The fraud detection electronic control unit includes a storage and a determination unit. The storage stores a first regulation for determining whether the frame transmitted from the electronic control unit is fraudulent. The determination unit determines whether the frame transmitted from the electronic control unit is fraudulent in pursuant to the first regulation. When a predetermined condition is satisfied, the storage acquires a second regulation retained by the electronic control unit and updates the stored first regulation.