Provided is an objective optical system employing a gradient index (GRIN) lens. A refractive index of the GRIN lens is changed in an axial direction and a direction perpendicular to the axial direction. The GRIN lens has a refractive index n satisfying the following equation:.function..times..times..times..times..times..times..times. ##EQU00001## where z is a distance from the center of the lens in the axial direction and r is a distance from the center of the lens in the direction perpendicular to the axial direction. Thus, an objective optical system with a high numerical aperture can correct aberration.