PURPOSE: To apply light flux whose diameter is larger than the number of openings on the whole surface of the ends of image fibers and to reduce an abberation by limiting the ranges of respective lens parameters in a short focus lens system for image fibers.

CONSTITUTION: When it is defined that a curvature is R, lens thickness and gap are t, the effective diameter of the lens is d and its refractive index is nd, the lens system is constituted so that respective lens parameters have values shown in Table I. The 3rd and 4th surfaces constitute an iris 8 and the number of openings is made to coincide with that of the image fibers arranged on an image formation surface. When a lens constituted as mentioned above is applied to the image fibers 1, especially when the lens having 2.7±0.1mm diameter is applied for image formation of the image fiber in which the difference of refractive indexes between the core and clad is 3.5W4.5%, the efficiency of image formation can be improved.