Disclosed is an optical sheet having an optical structure, such as a prism film or sheet, which is a constituent of a backlight unit. In the optical sheet, protrusions are formed on the optical structure layer, and thus light-collecting efficiency in a front direction is maintained, and a function of diffusing light to the front or the inclined surface is also exhibited, thus eliminating the need to additionally mount a diffusion film or a protection film, thereby obviating the use of a plurality of optical films, consequently making it possible to economically manufacture a backlight unit with improved productivity.