A surface illuminate device illuminates a liquid crystal display panel by diffused light produced through a diffusing plate disposed on the front of an illuminant section. A prism sheet having many triangular prisms is disposed on the exit side of the diffusing plate so that the longitudinal direction of each prism is nearly parallel to the plane of polarization of a polarizing plate placed on the prism sheet side of the liquid crystal display panel. Thus, the surface illuminant device can provide bright illumination suitable for the backlight of the liquid crystal display panel.