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 finite focal distance on the emission side of illumination light and have an aspheric face of a leftwardly and rightwardly symmetric sectional shape. 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))+AX.sup.4+BX.sup.5+CX.sup.6+ . . . (where R is the radius of curvature of a distal end vertex, K is a conic constant, and A, B, C, . . . are aspheric coefficients).