A reduction projection type aligner in a reduction projection exposing device for exposing a circuit pattern on a mask through a reduction projection lens onto a wafer by the step and repeat of the wafer, which comprises: a light source for irradiating coherent irradiation light, a reflection mirror for reflecting the coherent irradiation light irradiated from the light source, a detection optical system for detecting an interference pattern by optically causing interference between an alignment pattern reflection light obtained by entering the coherent irradiation light irradiated from the light source through the reduction projection lens to the alignment pattern portion of the wafer, which is then reflected at the alignment pattern portion and then passed through the reduction projection lens and a reflection light reflected at the reflection mirror, and means for aligning a mask and a wafer relatively by detecting the position of the wafer by the video image signals in the interference pattern detected by the detection optical system.