1. A retrieval apparatus comprising a Central Processing Unit (CPU), the CPU comprising: a search server that receives data comprising a search condition, wherein the search condition comprises a designated document designated by a person doing a search; a diagram information extraction server that extracts diagram information from image data of diagrams dotted throughout data of the designated document, wherein the diagram information comprises a feature amount of a diagram included in the designated document, an appearance order of the diagram, and an appearance position of the diagram; a similarity estimation server that compares the extracted diagram information with diagram information of a registered document that is registered in advance, and estimates a similarity between the data and the registered document; a scoring server that generates a search result capable of specifying a similar document that is related to the data, based on the estimated degree of similarity; a diagram information database storing diagram information of the registered document; wherein the similarity estimation server specifies a specified diagram that is similar in the designated document and the registered document, based on a result of the comparison; further estimates lines that are similar between both documents, on a basis of the specified diagram; and takes a line count of the estimated lines as the similarity estimated between the data and the registered document; and wherein the CPU further comprises a data update server that acquires data of the registered document from a database in which the registered document is stored, causes the diagram information extraction server to extract diagram information from the acquired data of the registered document, and updates the contents of the diagram information database, using the extracted diagram information.