To provide a projection optical system which reduces brightness unevenness and can assure a photographing range as wide as possible when imaged by using a digital single lens reflex camera and a microscope adapter unit for connecting imaging devices using the same.

The projection optical system has, successively from an object side, a first group GF including a lens element of a meniscus shape turning its convex face toward an object side and a second group GB including a lens element of a biconvex shape through an intermediate image position 11 and the magnification over the entire system satisfies the following conditional expression: 1<β<2, where β is the magnification over the entire system. The microscope adapter unit for connecting the imaging apparatus comprises having the projection optical system and a unit body section provided with a first juncture which is freely attachably and detachably connectable to a microscope body and a second juncture which is freely attachably and detachably connectable to the electronic imaging apparatus or an arbitrary optical unit.