To provide an information processing device that can perform highly accurate tampering detection of distinguishing between an alteration by an administrator and significant tampering. The information processing device acquires content from a web server in accordance with an acquisition request for the content by a browser terminal. The information processing device includes: a conversion unit (80) that converts the content to image information; an image information storage unit (84) that stores image information corresponding to the content; a similarity calculation unit (86) that calculates a degree of similarity between the image information obtained from the conversion unit (80) and the image information obtained from the image information storage unit (84); and a judgment unit (88) that judges whether or not the content has been tampered with, by comparing the degree of similarity with a preset threshold value.