To provide a direct-downward backlight device with the number of members reduced and capable of restraining luminance unevenness of a light-emitting face.

The direct-downward backlight device is provided with a plurality of linear light sources 2 arranged nearly in parallel, a reflecting plate 3 reflecting light from the linear light sources 2, and a light diffusion plate 1 receiving direct light from the linear light sources 2 and reflected light from the reflecting plate 3 from a light-incident face 1A and diffusing and irradiating the same from a light irradiation face 1B. An outer diameter of the linear light source is 5 mm to 20 mm. A plurality of triangular prisms 4 with an arithmetic mean roughness Ra (max) measured in a direction in which an obtained value becomes a maximum to be 3 μm to 1,000 μm are formed on the light-incident face 1B.

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