Automatic focusing apparatus which removes light reflected by a lower surface of a sample

An automatic focusing apparatus for an optical system which forms an image of a transparent thin sample and in which an objective lens is disposed at a position to oppose the sample, comprising:

• light source means for supplying light to measure a focusing state of said optical system;
• converting means for converting the light supplied from said light source means into a parallel beam parallel to an optical axis of said optical system, the parallel beam being incident on an eccentric portion of a pupil of said objective lens;
• separating means for separating light reflected by the sample from said optical system, the light reflected by the sample including light reflected by an upper surface of the sample and light reflected by a lower surface of the sample that are generated by reflecting the parallel beam, focused on the sample by said objective lens, by the upper and lower surfaces of the sample;
• coverging means, disposed on an optical path where the light reflected by the sample passes, for converging the light reflected by the upper surface of the sample and the light reflected by the lower surface of the sample at different predetermined positions;
• light-shielding means for shielding only the light reflected by the lower surface of the sample at one of the positions of a converged position of the light reflected by the upper surface of the sample and a converged position of the light reflected by the lower surface of the sample;
• and
• focusing state discriminating means for discriminating a focusing state of said optical system according to the light reflected by the sample from which the light reflected by the lower surface of the sample is removed.