By suitably correcting a secondary spectrum, a clear, bright optical image is obtained. Provided is a rigid-scope optical system including: an objective optical system; and at least one relay optical systems that are formed of positive front groups, middle groups, and back groups in this order from an entrance side and that reimage an optical image imaged at imaging planes at the entrance side onto imaging planes at an exit side, wherein axial chromatic aberration between two wavelengths is corrected by an optical system other than the diffractive optical element, and axial chromatic aberration between the two wavelengths and another wavelength is corrected by the diffractive optical element.