An optical system which is usable as either an ocular optical system or an imaging optical system, and which is satisfactorily corrected for both field curvature and comatic aberration for a field angle of up to about 120.degree. and for a pupil diameter of up to about 15 millimeters. The optical system includes a partial optical system (10) having a flat semitransparent surface (1) which is closer to a pupil plane (E), and a concave semitransparent surface (2) which is closer to an image plane (I). A refracting optical element (4) is disposed at the pupil plane (E) or image plane (I) side of the partial optical system (10) with an air spacing provided therebetween. Alternatively, the refracting optical element (4) may be cemented to the partial optical system (10). The refracting optical element (4) has a focal length f.sub.a which satisfies the condition of f.sub.a >0.