1. A reference image forming method in a pattern inspection apparatus for scanning a pattern formed on an object to be inspected on the basis of design data with a laser beam having a predetermined wavelength, focusing transmitted light passing through the object on a light-receiving element by using an objective lens, forming a real image from pattern information obtained from said light-receiving element, and comparing the real image with a reference image obtained by imaging the design data, thereby detecting a defect in the object, comprising the steps of: forming reference data by developing the design data as a pattern made of multilevel gradation values on pixels having a resolution higher than an inspection resolution; and forming a reference image by increasing or decreasing a width of each pattern of the reference data as a multilevel gradation pattern with a precision higher than the inspection resolution on the basis of an edge position of a corresponding pattern in the real image.