A multicasting system includes a delivery server for multicasting a content via at least one upper router and a plurality of lower routers, a plurality of client devices for playing the content multicast by the delivery server, an upper router controller for controlling the upper router and a lower router controller for controlling the plurality of lower routers. The client device includes a play history storage unit, an individual storage unit, a content retrieving unit, and a content playing unit. The delivery server includes a master storage unit, an optimizer optimizing the identifier and the session information of the content stored on the individual storage unit, and a content delivery unit. The upper router controller includes a session information retrieving unit, a book scheduling unit, and a schedule information notifier.