An optical sheet has a large number of cylindrical lens elements provided successively on one of principal faces thereof. The cylindrical lens elements have a hyperboloidal face or a paraboloidal face and have a finite focal distance on the emission side of illumination light. Where a Z axis is taken in parallel to a normal line direction to the optical sheet and an X axis is taken in a direction of the row of the cylindrical lens elements, a cross sectional shape of the cylindrical lens elements satisfies Z=X.sup.2/(R+ {square root over ( )}(R.sup.2-(1+K)X.sup.2)) (where R is the radius of curvature of a distal end vertex, and K is a conic constant).