Provides ability to analyze the readability of an image to be displayed on a screen as a web page, etc., and appropriately modify the image. It includes a rendering section for generating an image by rendering an HTML document; an image processing section for performing image processing on the image generated by the rendering section to simulate and evaluate how the image is viewed under a certain visual characteristic; and a result presentation section for locating an HTML element that needs to be modified in the HTML document to be processed based on the evaluation result of the image processing section 30 and for presenting the HTML element to a web page creator. The result presentation section also retrieves a modification method for the HTML element that needs to be modified from a symptom model storage section. A document modification processing section actually modifies the HTML document.