To provide a light guiding plate device capable of preventing the dissipation of the light entering the inside of a light guiding plate from a light source and improving the luminance in light emitting regions.

This light guiding plate device 10 includes the light source 12 and the light guiding plate 13 in which the light source 12 is arranged along its one side edge 13a, which is formed as a plate body of a rectangular shape in plane view provided with light emitting surfaces 14 on its front surfaces and reflection surfaces 15 on its rear surfaces and reflects the light received from the light source 12 by these reflection surfaces 15 so as to emit the light from the light emitting surfaces 14. Slits 16 are formed on the light transmission plate 13 along the side edges 13b, 13c, 13d not arranged with the light source 12 and in the position parted inward from these side edges 13b, 13c, 13d.