An optical system for observing a front object and an approximately lateral object, comprising: a front group (G f) having a reflective refractive optical element (L2 1) and negative refractive power, a rear group (G r) having a positive refractive power, wherein the reflective refractive optical element (L 21) includes a first surface (L 21a) formed on the front-object side and a second surface (L 21b) formed on the image side, and wherein the second surface (L 21b) has a second transmitting surface (L 21b1) and a second reflective surface (L 21b2), wherein the second transmitting surface (L 21b1) is formed with the center of the second transmitting surface (L 21b1) on the optical axis and the second reflective surface (L 21b2) is formed in the shape of a ring and around the second transmitting surface (L 21b1) and faces toward the front-object side; the first surface (L 21a) has a first transmitting surface (L 21a1) and a first reflective surface (L 21a2), wherein the first transmitting surface (L 21a1) is formed with the center of the first transmitting surface (L 21a1) on the optical axis and the first reflective surface (L 21a2) is formed in the shape of a ring and around the first transmitting surface (L 21a1) and faces toward the image side, and the reflective refractive optical element (L 21) further includes a third surface (L 21c) which is formed as a transmitting surface between the first and second surfaces (L 21a, L 21b), characterized in that, the front group (G f) further comprises an aperture stop and the rear group (G r) comprises a moving lens group (G m), wherein the front group (G f), the aperture stop and the rear group (G r), are arranged in order from the object side, where the moving lens group (G m) moves in the direction along the optical axis, wherein the optical system is configured to focus on the front object and the approximately lateral object while an observation angle of the optical system in the direction of the front object hardly changes when the moving lens group (G m) is moved to an extent that the refractive powers of the front and rear groups (G f, G r) hardly change relatively, wherein the moving lens (G m) group has positive refractive power. a front group (G f) having a reflective refractive optical element (L2 1) and negative refractive power, a rear group (G r) having a positive refractive power, wherein the reflective refractive optical element (L 21) includes a first surface (L 21a) formed on the front-object side and a second surface (L 21b) formed on the image side, and wherein the second surface (L 21b) has a second transmitting surface (L 21b1) and a second reflective surface (L 21b2), wherein the second transmitting surface (L 21b1) is formed with the center of the second transmitting surface (L 21b1) on the optical axis and the second reflective surface (L 21b2) is formed in the shape of a ring and around the second transmitting surface (L 21b1) and faces toward the front-object side; the first surface (L 21a) has a first transmitting surface (L 21a1) and a first reflective surface (L 21a2), wherein the first transmitting surface (L 21a1) is formed with the center of the first transmitting surface (L 21a1) on the optical axis and the first reflective surface (L 21a2) is formed in the shape of a ring and around the first transmitting surface (L 21a1) and faces toward the image side, and the reflective refractive optical element (L 21) further includes a third surface (L 21c) which is formed as a transmitting surface between the first and second surfaces (L 21a, L 21b), characterized in that, the front group (G f) further comprises an aperture stop and the rear group (G r) comprises a moving lens group (G m), wherein the front group (G f), the aperture stop and the rear group (G r), are arranged in order from the object side, where the moving lens group (G m) moves in the direction along the optical axis, wherein the optical system is configured to focus on the front object and the approximately lateral object while an observation angle of the optical system in the direction of the front object hardly changes when the moving lens group (G m) is moved to an extent that the refractive powers of the front and rear groups (G f, G r) hardly change relatively, wherein the moving lens (G m) group has positive refractive power.