We propose methods of searching Web pages that are “semantically” regarded as “siblings” with respect to given page examples. That is, our approach aims to find pages that are similar in theme but have different content from the given sample pages. We called this “sibling page search”. The proposed search methods are different from conventional content-based similarity search for Web pages. Our approach recommends Web pages whose “conceptual” classification category is the same as that of the given sample pages, but whose content is different from the sample pages. In this sense, our approach will be useful for supporting a user's opportunistic search, meaning a search in which the user's interest and intention are not fixed. The proposed methods were implemented by computing the “common” and “unique” feature vectors of the given sample pages, and by comparing those feature vectors with each retrieved page. We evaluated our method for sibling page search, in which our method was applied to test sets consisting of page collections from the Open Directory Project (ODP).