Superposition measuring apparatus, superposition measuring method, and superposition measuring system

When a scanning electron microscope is used to measure a superposition error between upper-layer and lower-layer patterns, an SN of the lower-layer pattern may often be lower, so that when simple frame adding processing is used, the adding processing needs to be performed many times. Further, in an image obtained through such simple adding processing, contrast may not be optimal for both the upper-layer and lower-layer patterns. In a superposition measuring apparatus and superposition measuring method that measure a difference between a position of an upper-layer pattern and a position of a lower-layer pattern by using an image obtained by irradiation of a charged particle ray, portions of images having contrasts optimized for the respective upper-layer and lower-layer patterns are added to generate a first added image optimized for the upper-layer pattern and a second added image optimized for the lower-layer pattern, and the difference between the position of the upper-layer pattern identified by using the first added image and position of the lower-layer pattern identified by using the second added image is calculated.