1. A document monitor device including at least one central processing unit (CPU), the 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, using the at least one CPU, selects a target document from the documents managed in the document management device based on document selection conditions and 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, using the at least one CPU, extracts features of the target image acquired by the acquisition section; a similarity level calculation section that, using at least one CPU, determines 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 of the target image extracted by the extraction section; and a detection information output section that, when the similarity level determined by the similarity level calculation section is a prescribed value or over, outputs information indicating that a similar image is detected, wherein the similarity level calculation section separates the documents managed by the document management device into confidential documents whose similarity is to be determined at real-time, and remaining documents whose similarity is to be determined at an idle time, and the selection section selects the target document from the confidential documents in real-time when the target image is acquired; and wherein, during an idle time, the similarity level calculation section compares features of the remaining documents with the features of the target image and determines similarity levels indicating a similarity level of the features, and the detection information output section, when the similarity level determined by the similarity level calculation section is a prescribed value or over, outputs information indicating that a similar image is detected.