the invention relates to an epi - illumination optical system for microscopes which relies upon a köhler illumination system using a collective lens array , having simplified adjustment function capable of adjusting the image position of a light source to the back focal position of each microscope objective lens in operable association with microscope objective lens replacement . the invention provides an epi - illumination optical system for microscopes , with an objective lens serving as a condenser lens , which comprises a light source , a collimate lens , and a lens array . the epi - illumination optical system further comprises a projection optical system adapted to project an image of light source formed by collective lens array . the projection optical system comprises a partial optical system movable in an optical axis direction , so that the imaging position for the image array of light source is adjustable to the back focal position of the objective lens .