A server deletes a mask object in a structured text as a part of a shared text. A client issues a mask object constituting each layout object of the structured text, according to the authority of the client, and adds the mask object to the shared text data from which the mask object received from the server is deleted. With this arrangement, information written in the shared text is made unreadable according to the authority of the user of the client, thereby achieving access control to a part of the shared text.