Abstract: According to a focus monitoring method, an exposure mask on which a focus monitor pattern comprising at least two types of pattern groups is formed is prepared. A pattern group A of the at least two pattern groups is illuminated with illumination light while a barycenter of an illumination light source of illumination optics is in an off-axis state. At least a pattern group B of the at least two pattern groups is illuminated with illumination light while the barycenter of the illumination light source is in an on-axis state. A positional deviation between the pattern groups A and B transferred onto a substrate is measured. An effective focus position can be monitored from this positional deviation.