A monitoring apparatus includes: an acquisition unit sequentially receiving messages on a network; a first calculation unit calculating a difference between data values of two continuous messages; a second calculation unit calculating a difference between reception time points of two continuous messages; a first determination unit determining whether a received message is an abnormal message based on the difference calculated by the first calculation unit or the second calculation unit; a second determination unit determining whether a received message is a suspicious message based on the difference calculated by the first calculation unit or the second calculation unit; and a recording unit recording, when the second determination unit determines that the received message is the suspicious message, as a history, data values and reception time points of the suspicious message and a predetermined number of messages received before and after reception of the suspicious message.