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