This paper presents an optical apparatus for measuring small hole diameter. The apparatus, basing on the principle of geometrical optics, enables to measure diameters of through holes with high aspect ratios. The measuring range is 0.1-1mm. Four light beams made up by concyclic four pinholes incident onto hole wall with a large incident angles and their reflected beams form four images on a charge coupled device. From optical relations and picture processings about the images, the value of the diameter is obtained. Being employed ferrules of the optical fiber connecter, etc., as test specimens, some problems about measuring conditions and data processings are experimentally examined and solved. According to actual measurements, the measuring accuracy is estimated to be 2.3 μm(standard deviation) and, under a special condition, 1.4 μm(standard deviation). It is also found that the apparatus can measure specimens without mirror surfaces well.