A memory sharing apparatus includes a server, a host and a client. The server includes a shared page which is an entity of a shared memory, a share setting page which is data in which an index value of each shared page is collected, and a grant table in which a page frame number of each share setting page and the index value are stored so as to correspond to each other. The host includes a database in which the index value in the grant table is managed. The client includes the shared page and a shared page area to which the shared page is mapped, and a share setting page area to which the share setting page is mapped.