The micro lens array illumination device has an advantage on transmittance, but it also has the problem of the generation
of diffraction pattern. Well-known method for reducing diffraction is adding randomness to array structure (random
structure), but there are many choices to do it. In this study we examined the randomness with a scope that is realizable
by diamond machining yet with good productivity. As the result, we have found that the diffraction pattern can be
reduced sufficiently by adding randomness of some 10% of its lattice constant to spatial configuration of periodic lattice
of micro lens array. In addition, we have also examined controlling the illuminance distribution, by taking advantage of
high form accuracy which is special features of diamond machining.