The monitoring device includes a receiver and a processor. The receiver receives a frame from a communication network. The processor performs a first determination that determines whether the frame is illegal based on a result of message authentication for the frame and a second determination that determines whether the frame is illegal based on a state of the frame and a predetermined rule. In addition, the processor executes, in accordance with a combination of a result of the first determination and a result of the second determination, at least one of processing for the frame, processing for a transmission source device of the frame, change of contents to be notified to an external device, and change of priority of notification to the external device.