PURPOSE: To well correct the chromatic aberration, curvature of field and astigmatism of an observation optical system in particular by using a single biconvex lens of which at least either face of the face on the side of the eye to be examined and the face on an observer side is formed to an aspherical face and which satisfies specific conditions.

CONSTITUTION: The inverted image lens consists of the single biconvex lens. At least either face of the face (first face) on the side of the eye to be examined and the face (second face) on an observer side is formed to the aspherical face. The lens described above satisfies the conditions 65<νd, nd<1.67, where νd and nd denote the Abbe number and refractive index for the (d) line of the biconvex lens. Further, the second face forms the aspherical face and is preferably so set as to satisfy the condition equations -0.85≤R2/f≤-0.4, 3.2≤(dx/ dy).f/y≤-0.5. Here, R2 denotes the radius of paraxial curvature of the aspherical face and (f) denotes the focal length. Also, (x) denotes a displacement in an optical axis direction with the vertex of the aspherical face as an origin and (y) denotes the displacement in a direction perpendicular to the optical axis.