A rigid-scope optical system comprising: an objective optical system; characterized by: a plurality of N relay optical systems (10, 20, 30, 40, 50) that reimages an optical image imaged at a primary imaging plane (P1-P5) at the entrance side onto a secondary imaging plane (P2-P6) at an exit side; wherein the first (N-1) relay optical systems (10, 20, 30, 40) are formed identical of a combined optical lens system formed of a front group (FG1-FG4) having positive power, a positive biconvex middle group (MG1-MG4), and a back group (BG1-BG4) having a positive power in this order and are disposed in an ordered sequence after the objective optical system, wherein the positive biconvex middle group (MG1-MG4) of each N-1 relay optical systems (10, 20, 30, 40) is formed of a cemented lens comprising a low-dispersion convex lens and a high-dispersion concave lens, and wherein the middle group of the Nth relay optical system disposed after the N-1 relay optical systems (10, 20, 30, 40) at an extreme exit side of the rigid-scope optical system is provided with a diffractive optical element (Ldoe) having a diffractive surface and at least one biconvex lens, and wherein the diffractive optical element (Ldoe) is formed by combining two optical elements, which are formed of glass materials having different refractive indices and Abbe numbers from each other and have diffractive surfaces (Pdoe) at one of their surface such that the diffractive surfaces (Pdoe) face each other, and wherein the diffractive optical element (Ldoe) receives an incident light beam, at the diffractive surface, whose axial chromatic aberration between at least two wavelengths corresponding to the C-line that corresponds to red and the F-line that corresponds to blue contained in the incident light beam has been corrected by the positive biconvex middle groups (MG1-MG4) of the first N-1 relay optical systems (10, 20, 30, 40), and the diffractive optical element (Ldoe) corrects axial chromatic aberration related to another wavelength corresponding to the e-line which corresponds to green contained in the light beam. an objective optical system; a plurality of N relay optical systems (10, 20, 30, 40, 50) that reimages an optical image imaged at a primary imaging plane (P1-P5) at the entrance side onto a secondary imaging plane (P2-P6) at an exit side; wherein the first (N-1) relay optical systems (10, 20, 30, 40) are formed identical of a combined optical lens system formed of a front group (FG1-FG4) having positive power, a positive biconvex middle group (MG1-MG4), and a back group (BG1-BG4) having a positive power in this order and are disposed in an ordered sequence after the objective optical system, wherein the positive biconvex middle group (MG1-MG4) of each N-1 relay optical systems (10, 20, 30, 40) is formed of a cemented lens comprising a low-dispersion convex lens and a high-dispersion concave lens, and wherein the middle group of the Nth relay optical system disposed after the N-1 relay optical systems (10, 20, 30, 40) at an extreme exit side of the rigid-scope optical system is provided with a diffractive optical element (Ldoe) having a diffractive surface and at least one biconvex lens, and wherein the diffractive optical element (Ldoe) is formed by combining two optical elements, which are formed of glass materials having different refractive indices and Abbe numbers from each other and have diffractive surfaces (Pdoe) at one of their surface such that the diffractive surfaces (Pdoe) face each other, and wherein the diffractive optical element (Ldoe) receives an incident light beam, at the diffractive surface, whose axial chromatic aberration between at least two wavelengths corresponding to the C-line that corresponds to red and the F-line that corresponds to blue contained in the incident light beam has been corrected by the positive biconvex middle groups (MG1-MG4) of the first N-1 relay optical systems (10, 20, 30, 40), and the diffractive optical element (Ldoe) corrects axial chromatic aberration related to another wavelength corresponding to the e-line which corresponds to green contained in the light beam.