To provide a plane illumination device for a front light which can be made small size and in which the image display is easily viewed without having bright lines or a Moire fringe on the light-guiding plate.

The illumination device comprises a light guiding plate 1 formed with micro prisms on the viewing face side, a light introducing plate 2 that is arranged along one side of the outer frame of this light guiding plate 1, and a light source 3 that is arranged opposed to one end side of this light introducing plate 2, and the light source 3 is contained in the outer frame of the light guiding plate 1, and also the direction of the pitch of the waveform of the micro prisms 1a is formed so as to be in acute angle to the optical axis of the light from the light source 3.