A document monitor device, which includes: a confidential information storage section that stores confidentiality level information of documents managed in a document management device associated with identification information of each of the documents; a feature storage section that stores features of the documents associated with the identification information; a selection section that selects a target document from the managed documents based on the confidentiality level information; an acquisition section that acquires a target image to be processed by an image processing device; an extraction section that extracts features of the target image; a similarity level calculation section that calculates a similarity level between the features stored in association with the identification information of the selected target document and the extracted features; and a detection information output section that, when the similarity level is a prescribed value or over, outputs information indicating that a similar image is detected.