Seunghee BahngThis study examines a role play as the device of the play-within-a-play in Shakespeare's Twelfth Night. Shakespeare chooses to replace reality with illusion employing the device of the play-within-a-play which transforms unreal things into real ones. It creates confusion and pleasure, in which the false things look real, and vice versa. He uses role players as well as the play itself to transform reality into illusion. It is also play-within-a-play without the form of a play. We can find the same effect of the play-within-a-play in the stage audience in Twelfth Night. Viola's disguise and Malvolio's transformation are included in a kind of play-within-a-play. We can see the stage audience which is watching Viola's and Malvolio's role play. Viola's stage audience is Olivia, Orsino, Antonio, and Andrew; Malvolio's is Maria, Sir Toby, and Fabian. They confuse the play and reality, but at the same time they distinguish illusion from reality. The difference between appearance and reality cause Viola's stage audience to be confused. They can't separate illusion from reality. The game which gulls Malvolio amuses his stage audience and helps them discern the difference between play and reality. This kind of plays, however, does not last long. As the stage audience's reality turns out to be illusion, play and reality are switched. From the theatre audience's point of view, the play which the stage audience is watching becomes reality. The stage audience's shift from confusion to realization is moved into the theatre audience. They realize that the reality which they experience could also be an illusion as the play represents. In Twelfth Night we enjoy a game of illusion and reality which evokes confusion and pleasure, and we find that what it reflects is nothing but the illusion of our own reality.