A telescope optical system TL comprising, in order from an object side: an objective lens 1; an erecting prism 2; and an eyepiece 3; the objective lens 1 comprising, in order from the object side, a first lens group G1 having positive refractive power, a second lens group G2 having negative refractive power, and a third lens group G3 having positive refractive power, focusing being carried out by moving the second lens group G2 along an optical axis, and an image position being movable by moving the third lens group G3 in a direction perpendicular to the optical axis, thereby providing a telescope optical system having optimum optical performance for a telescope.