To provide a backlight wherein directivities in longitudinal and lateral directions are made equal to each other.

Prisms each consisting of two prism surfaces 4a and 4b parallel to each other in a direction X are formed continuously in a direction Z on the lower surface side of a prism sheet 4 of the backlight 1. The prism surface 4a nearer to a light source of the two prism surfaces is a flat surface and the prism surface 4b faced on the side opposite to the light source is a curved surface whose longitudinal section in the length direction is formed in a recessed shape on the light source side. When emitted light from a light guide plate 3 of the backlight 1 is traced, the light goes in a direction Y to be an irradiation direction while expansion in the direction Z of the light is made wider because of the recessed surface of the prism surface 4b when the light is reflected by the prism sheet 4 as shown in the figure.