by the focusing servo function of the apparatus that is when the object 25 is not on the holding base 24 the pin 17 is at a position several micrometers downward with respect to the focusing position by gravity when the object 25 is on the holding base 24 the lower end of the pin 17 contacts the object surface 18 then the optical system moves upward or downward by the focusing servo function so that the objective lens 6 moves to the focusing position therefore the distance from the objective lens 6 to the upper surface of the leaf spring 7 is maintained constant the z coordinate measurement of the object surface 18 is carried out by the measuring light fz1 which is condensed on the surface of the leaf spring 7 the measuring light fz1 and the laser beam g are condensed on approximately the same position of the leaf spring 7 the principle of measuring the object surface 18 in xyz coordinate utilizing a he ne zeeman laser beam f will be described below a part of the laser beam f irradiated from a he ne frequency stabilizing zeeman laser 28 which oscillates at two frequencies f1 and f2 passes through a first half mirror 29 and are separated from each other by a.