An abnormality detection apparatus for a mobility entity and for detecting an abnormality in a network system is provided. The network system includes a first network and a second network that use different communication protocols. A first communication circuit receives state information indicating a state of the mobility entity. The state information is acquired from the second network. A second communication circuit transmits and receives a first frame according to a communication protocol used in the first network. A memory stores an abnormality detection rule. A processor detects, based on the state information and the abnormality detection rule, whether a control command included in the first frame received by the second communication circuit is abnormal. In a case where the control command is abnormal, the processor prohibits the control command from being transmitted.