The present invention relates, in a projection aligner wherein a mask and a wafer are held proximate and wherein a circuit pattern depicted on the mask is transferred onto the wafer, to a method of detecting the respective positions of the mask and the wafer for the relative positioning between the mask and the wafer. To the end of dispensing with the withdrawal of a microscope objective in such a way that the objective of a microscope for detecting the mask and the wafer and projection light, for example, an X-ray are prevented from interfering, thereby to achieve the enhancement of throughput and to permit the detection of the positions of the mask and the wafer even during projection, the present invention consists in that the objective of the microscope is inclined with respect to a perpendicular to the plane of the mask or the plane of the wafer being a plane to-be-detected, so as not to interfere with the projection light, for example, the X-ray, whereby the circuit pattern can be transferred while the relative positions of the mask and the wafer are being detected.