An objective lens used for a microscope includes a lens(es), an iris diaphragm and a tubular body. The lens(es),which is provided in a manner facing an object, transmits light reflected from a measuring surface of the object. The iris diaphragm, which is provided behind the lens(es), changes an aperture diameter of a light-transmissive aperture of a light-transmissive surface that is substantially orthogonal to a main optical axis of the light transmitted through the lens(es). The tubular body, which is detachably mounted on a revolving nosepiece of a trunk that includes a zoom imaging lens(es) for forming an image from the light transmitted through the light-transmissive aperture, holds the lens(es) and the iris diaphragm.