1. A computer executable document search method for searching for a document, comprising: generating, based on user information represented as a group hierarchy having a user identifier and one or more groups joined by one or more operators, an inclusion relationship expression including an operator that designates an inclusion relationship for access authority; combining the inclusion relationship expression with a received search expression to generate a combined search expression; for searching at a private level, using the combined search expression to generate a first search expression using the received search expression and specifying the user identifier without specifying the one or more groups; and for searching at a shared level, using the combined search expression to generate a second search expression using the received search expression and specifying the one or more groups without specifying the user identifier.
1. A computer executable document search method for searching for a document, comprising: generating, based on user information represented as a group hierarchy having a user identifier and one or more groups joined by one or more operators, an inclusion relationship expression including an operator that designates an inclusion relationship for access authority; combining the inclusion relationship expression with a received search expression to generate a combined search expression; for searching at a private level, using the combined search expression to generate a first search expression using the received search expression and specifying the user identifier without specifying the one or more groups; and for searching at a shared level, using the combined search expression to generate a second search expression using the received search expression and specifying the one or more groups without specifying the user identifier. 6. The document search method according to claim 1 , wherein the access authority is set for each of a private level to which a user has access, a domain level including a plurality of users, and an open domain level including a plurality of domain levels to enable secure search.