To continuously detect a suspicious person, and to provide information for identifying the suspicious person, a monitoring device for monitoring an object existing in a detection area includes: a detection unit for detecting existence of the object in the detection area; an authentication information acquisition unit for acquiring authentication information held by the detected object; a registered authentication information storage unit for storing registered authentication information on an object admitted into the detection area; a determination unit for acquiring, based on the authentication information and the registered authentication information, a number of registered authentication objects and a number of unregistered authentication objects, for setting an alert mode when the detection unit detects the existence of the object, and when the number of registered authentication objects is zero, and for determining a suspicious object based on the number of registered authentication objects, the number of unregistered authentication objects, and the alert mode; and an alarming/notifying unit for notifying an alarm based on a result of determination by the determination unit.