PURPOSE: To provide high optical performance while composing one relay of an image transmitting optical system used for a rigid endoscope from two lenses.

CONSTITUTION: In an optical system of almost unmagnified image, two positive leness composition the lens system are radial type ununiform lenses represented by the equation; N(r)=N00+N10r2+N20r4+..., where r is a distance in the radial direction from the optical axis, N(r) is the refractive index of reference wavelength on the point of radial (r), N00 is the refractive index of a reference wavelength on the optical axis and N10, N20... are the second and fourth order refractive index distribution coefficients of reference wavelength. At least on of the radial type ununiform lenses satisfy the following conditions: (1) N10<0, (2) 1/V10<2/V00, where V00, V10 represent a first order and a second order dispersions of the radial type ununiform leness.