Illuminating a hologram by a non-divergent narrow laser beam is an approach of reconstruction that holographer often abopt. However, prior to the thesis [1], the relating theoretical analysis of it has not yet been found in any technical literature, In the said thesis an analysis is made in the case that the hologram is illuminated by a narrow laser beam under the condition of paraxial approximation, and here the much more common phenomena in the case of non-paraxial condition is discussed, and the wave length of the illuminating laser beam is not the same as that of the recording laser. The theoretical result obtained agrees well with the experiment.