A monitoring device has a detection target person storage part in which a feature of a face of each detection target person is stored, an image processor that processes images captured with a plurality of imaging devices having different imaging areas, and detects the image in which the detection target person stored in the detection target person storage part is captured, a detection information storage part in which detection information is stored, the detection information including the detection target person, imaging area, and imaging date and time with respect to the image in which the detection target person detected by the image processor is captured, and a reliability calculator that calculates a degree of detection reliability in the image processor based on a time-space rationality, the time-space rationality being determined from a plurality of pieces of detection information on each detection target person.