Reflected light caused by the state of the surface of a wafer, a foreign material, or a defect is superimposed on a haze frequency component caused by the type and thickness of a film or a surface irregularity. In order to detect a haze frequency component caused by a haze present on the surface of an object to be inspected, light propagating from the object to be inspected is detected and converted into an electric signal. The electric signal is sampled at a predetermined sampling time interval and converted into digital data. A frequency component caused by a foreign material, a defect or the like is separated from the digital data to ensure that a haze frequency component is selected. The haze frequency component is caused by a stain attached to the surface of the wafer, hazy tarnish, a surface irregularity or the like.