To correct such aberration as chromatic aberration, curvature of field, and spherical aberration well with an inexpensive lens system constituted of a small number of lenses, by constituting an eyepiece of one gradient index lens, whose refractive index is changed in accordance with distance in the radial direction from an optical axis and whose medium power is negative.

The eyepiece is constituted of a radial type GRIN lens, and the medium power of the eyepiece is negative, so that the curvature of field can be compensated without the negative power on a lens surface. In this case, since the respective surfaces have positive power and the respective surfaces can share the power, the amount of aberration occurring on each surface can be made small. That is, the medium has the negative power, so that the various kinds of aberration including the curvature of field can be compensated well with the small number of lenses. Thus, the chromatic aberration and the curvature of field can be corrected even with one radial type GRIN lens.