1. A retrieval apparatus comprising: a data receiving unit that receives data to serve as a search condition; a diagram information extraction unit that extracts diagram information included in the data; a similarity estimation unit 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; and a scoring unit that generates a search result capable of specifying a similar document that is related to the data, based on the estimated degree of similarity.