An apparatus for measuring an alignment accuracy between overlaid alignment marks formed to each of alignment mark portions on every plural chip units or exposure units arranged on a substrate to be measured includes an XY stage movable X and Y directions while mounting the substrate, an illumination system for illuminating each of the alignment mark portions, a detecting system having a lens for collecting a reflection light, a focusing system for focusing the reflection light, a scanning system for scanning a reflection light image and an image sensor receiving reflection light image for conversion into an image signal. An alignment accuracy calculator measures the alignment accuracy between the overlaid alignment marks.