4. A system comprising: a server that stores therein shared text data encoded in a structured document format, and that is connected to a plurality of clients, the server comprising: a structure analyzing unit to perform structure analysis of the shared text data; a position-and-length specifying unit to specify a position and a length of a codestream of a mask object structuring each layout object of a structured text based on a result of the structure analysis; an updating unit to update at least a length of the codestream of the mask object; and an outputting unit to output the shared text data updated by the updating unit to an access-requested client; and the access-requested client that is connected to the server and that observes shared text data encoded in a structured document format stored in the server, the access-requested client comprising: an authenticating unit to check authority of a user; a mask issuing unit to issue a mask object constituting each layout object of a shared text according to the authority of the user; a structure analyzing unit to perform structure analysis of the shared text data from which the codestream of the mask object received from the server is deleted; an adding unit to add the codestream of the mask object issued by the mask issuing unit to the shared text data from which the codestream of the mask object received from the server is deleted based on a result of the structure analysis; a client updating unit to update reference information of the shared text data to refer the codestream of the mask object added by the adding unit; and a client outputting unit to output the shared text data updated by the client updating unit.
4. A system comprising: a server that stores therein shared text data encoded in a structured document format, and that is connected to a plurality of clients, the server comprising: a structure analyzing unit to perform structure analysis of the shared text data; a position-and-length specifying unit to specify a position and a length of a codestream of a mask object structuring each layout object of a structured text based on a result of the structure analysis; an updating unit to update at least a length of the codestream of the mask object; and an outputting unit to output the shared text data updated by the updating unit to an access-requested client; and the access-requested client that is connected to the server and that observes shared text data encoded in a structured document format stored in the server, the access-requested client comprising: an authenticating unit to check authority of a user; a mask issuing unit to issue a mask object constituting each layout object of a shared text according to the authority of the user; a structure analyzing unit to perform structure analysis of the shared text data from which the codestream of the mask object received from the server is deleted; an adding unit to add the codestream of the mask object issued by the mask issuing unit to the shared text data from which the codestream of the mask object received from the server is deleted based on a result of the structure analysis; a client updating unit to update reference information of the shared text data to refer the codestream of the mask object added by the adding unit; and a client outputting unit to output the shared text data updated by the client updating unit. 6. The server according to claim 4 , wherein shared text data encoded in the structured document format is a file prescribed by the International Standard IS15444-6.