PURPOSE: To easily carry out the initial setting operation of a work by constituting the apparatus so that the position to be measured of the work can be set and guided into the vicinity of an observing optical axis in reference to guide light.

CONSTITUTION: The guide light emitted from a guide-light means 23 is condensed in a spot shape at a position, which is observed at an observing part 6, with an observing optical system 21. There fore, the light is condensed in this spot shape. When the guide marking spot, which is condensed in this spot shape, is alinged with a position to be measured P of a work 10 by moving, e.g. the side of the work 10, the position to be measured P is observed at the observing part 6. When, e.g. the aperture to be measured is small only the shadow is seen when the inside is observed from the observing part 6 at the initial position setting of the work 10 and when the position detecting part P is hard to be caught by the observing part becuase of the reason described below, at first, the spot position based on the means 23 is visually aligned with the position P or the work 10 is moved to the vicinity of the position P and the fine adjustment of the work 10 is performed at the observing part 6. The reason of the difficulty is that in which direction among the leftward, right ward, forward and backward directions the work 10 should be moved to come to the optical-axis position of the observing part 6.