By suitably correcting a secondary spectrum, a clear, bright optical image is obtained. Provided is a rigid-scope optical system (100) including: an objective optical system (1)； and at least one relay optical systems (10 to 50) that are formed of positive front groups (FG1 to FG5), middle groups (MG1 to MG5), and back groups (BG1 to BG5) in this order from an entrance side and that reimage an optical image imaged at imaging planes (P1 to P5) at the entrance side onto imaging planes (P2 to P6) at an exit side, wherein a diffractive optical element (Ldoe) provided in the middle group (MG5) of one of the relay optical systems (50) receives an incident light beam, at a diffractive surface Pdoe, whose axial chromatic aberration between two wavelengths has been corrected by other optical systems (10 to 40), and the diffractive optical element (Ldoe) corrects axial chromatic aberration related to another wavelength contained in the light beam.
