To discriminate a suspicious person who intrudes into a facility by monitoring the facility with a simple configuration and at a low cost.

A suspicious person deciding device is provided with a difference means 221 for entirely or partially difference images adjacent in the vicinity of at least a prescribed interval, obtained by imaging a prescribed area, an abnormality deciding means 222 for comparing the difference with a prescribed threshold, deciding that abnormality exists on the basis of the magnitude of the difference after change occurs, considering that the change occurs in the case the difference is equal to or greater than the prescribed threshold, and deciding that no abnormality exists in the case the difference is not equal to or greater than the prescribed threshold, and a deciding means 223 for deciding that a suspicious person exists in the prescribed area in case that decision that abnormality exists occurs by the prescribed number of times within a prescribed time.