A telescope optical system having an objective lens system and an eyepiece lens system; wherein: the objective lens system includes a multi-layer-type diffractive optical element (PFo), and cemented lenses (So1) provided with a lens having positive refractive power and a lens having negative refractive power; the eyepiece lens system includes a multi-layer-type diffractive optical element (PFe), and cemented lenses (Se1) provided with a lens having negative refractive power and a lens having positive refractive power; and the condition represented by: 2 ‰¤ |(Po/FNO)/{Pe/( ¸ × m)}| ‰¤ 15 is satisfied, where Ko represents the power of the objective lens system, Kodoe represents the power of the diffractive optical element (PFo) of the objective lens system, and Po is defined as Po = Kodoe/Ko; Ke represents the power of the eyepiece lens system, Kedoe represents the power of the diffractive optical element (PFe) of the eyepiece lens system, and Pe is defined as Pe = Kedoe/Ke;, and FNO represents the F-n
