Simple focus monitoring method has been successfully developed by application of a special illumination aperture, which generates oblique illumination beam. By this method, very high sensitive focus monitoring has been achieved in a current stepper. In the stop of the illumination aperture, an opening is located at eccentric position near pupil edge. Then, illumination beam obliquely incidents to mark pattern on mask. Because of this configuration of illumination beam, imaging is carried out with oblique beams on wafer. As a result, imaging becomes non-telecentric. That is, image formed by this illumination laterally shifts almost proportional to focal deviation. To measure the lateral pattern shift, box-in-box mark is formed by double exposure. Inner box is formed by the oblique illumination in the first exposure and outer box is formed by conventional low coherent illumination in the second exposure overlaying inner box by stepping of wafer. Then, relative displacement of inner box to outer box is measured by commercially available overlay measurement system. Since sine of landing angle of imaging beams is approximately NA*sigma, which is over approximately 0.50 in a current stepper, the focus sensitivity, which is defined by a ratio of lateral pattern shift per unit defocus, may become approximately 0.50. Because resolution of lateral pattern shift is approximately 2 nm in current overlay measurement, the resolution of focus sensing becomes very high of approximately several nm.