PURPOSE: To obtain a highly reliable monitor device which issues an alarm without overlooking any abnormal condition which shows little image change by updating a background while keeping the background image accompanying only a current background even when an image changes.

CONSTITUTION: Plural background memories 31 and 32, an image change detecting part 4 which obtains a change image between the picture image and a processing image, a decision process part 5 which decides whether an abnormal condition has occurred or not by considering the above result and a background image update control part 6 which controls updating of a background so as not to take in a background image at the time of abnormality are provided.