Monitoring device and interference detection method

A monitoring device that includes an imaging unit which images an image, a turning unit which turns an angle of the imaging unit between a plurality of predetermined positions, an interference detection unit which detects an interference action based on an image sequence imaged in one of the plurality of predetermined positions. A past image storage unit stores an image imaged before the imaging unit turns, and a turning-period interference detection unit detects an interference action in a period the imaging unit turns, using the image imaged in a predetermined position before the imaging unit turns and the image stored in the past image storage unit, when the imaging unit turns after the past image storage unit stores the image, and turns back to a predetermined position before its turn.