A monitoring device configured to monitor a network to which plural controllers are connected which includes a decoder configured to extract target data belonging to a target data group from data received from the plural controllers, a first comparator configured to determine whether an immutable part of the target data is known or anomalous, a second comparator configured to determine whether a reception interval of the target data group is normal or anomalous, a third comparator configured to determine whether the number of the target data having been received and included in the target data group is normal or anomalous, and determine whether each reception interval between the target data is normal or anomalous, and a warning counter configured to individually count the number of anomalies determined by the first comparator, the number of anomalies determined by the second comparator, and the number of anomalies determined by the third comparator.