An optical system for observation of a front-direction object and a substantially-lateral-direction object, the optical system comprising, in order from a front-direction-object side: a front group (G f) with a negative refracting power having an optical element (L 21); an aperture stop; and a rear group (G r) with a positive refracting power having a moving lens component (L 31) that is movable along an optical axis; wherein the optical element (L 21) has: a first face (L 21 a) that is formed on the front-direction-object side and that has a first transmitting surface (L 21 a 1) formed with the optical axis being at a center thereof and a first reflecting surface (L 21 a 2) annularly formed around the first transmitting surface (L 21 a 1) and directed toward an image side, and a second face (L 21 b) that is formed on the image side and that has a second transmitting surface (L 21 b 1) formed with the optical axis being at a center thereof and a second reflecting surface (L 21 b 2) annularly formed around the second transmitting surface (L 21 b 1) and directed toward the front-direction-object side, and a third face (L 21 c) formed as a transmitting surface on a full circumferential surface of the optical element (L 21) between the first face (L 21 a) and the second face (L 21 b), characterized in that a movement of the moving lens component (L 31) changes a refracting power ratio of the front group (G f) to the rear group (G r), to enlarge or shrink, in an observation region, a region in which an image of the front-direction object is formed; and the following condition is satisfied: 1.05 < f r_c / f r_w < 1.45 where f r_w is a focal length of the rear group (G r) in a state where the region in which the image of the front-direction object is formed is in a central region of the observation region, and the substantially lateral-direction object is formed in an annular region thereon and such that simultaneous observation of the front-direction object and the substantially-lateral-direction object is made, and f r_c is a focal length of the rear group (G r) in a state where the region in which the image of the front-direction object is formed is enlarged in the entire observation region, such that enlarged-view observation of the front direction object is made. a front group (G f) with a negative refracting power having an optical element (L 21); an aperture stop; and a rear group (G r) with a positive refracting power having a moving lens component (L 31) that is movable along an optical axis; wherein the optical element (L 21) has: a first face (L 21 a) that is formed on the front-direction-object side and that has a first transmitting surface (L 21 a 1) formed with the optical axis being at a center thereof and a first reflecting surface (L 21 a 2) annularly formed around the first transmitting surface (L 21 a 1) and directed toward an image side, and a second face (L 21 b) that is formed on the image side and that has a second transmitting surface (L 21 b 1) formed with the optical axis being at a center thereof and a second reflecting surface (L 21 b 2) annularly formed around the second transmitting surface (L 21 b 1) and directed toward the front-direction-object side, and a third face (L 21 c) formed as a transmitting surface on a full circumferential surface of the optical element (L 21) between the first face (L 21 a) and the second face (L 21 b), a first face (L 21 a) that is formed on the front-direction-object side and that has a first transmitting surface (L 21 a 1) formed with the optical axis being at a center thereof and a first reflecting surface (L 21 a 2) annularly formed around the first transmitting surface (L 21 a 1) and directed toward an image side, and a second face (L 21 b) that is formed on the image side and that has a second transmitting surface (L 21 b 1) formed with the optical axis being at a center thereof and a second reflecting surface (L 21 b 2) annularly formed around the second transmitting surface (L 21 b 1) and directed toward the front-direction-object side, and a third face (L 21 c) formed as a transmitting surface on a full circumferential surface of the optical element (L 21) between the first face (L 21 a) and the second face (L 21 b), characterized in that a movement of the moving lens component (L 31) changes a refracting power ratio of the front group (G f) to the rear group (G r), to enlarge or shrink, in an observation region, a region in which an image of the front-direction object is formed; and the following condition is satisfied: 1.05 < f r_c / f r_w < 1.45 where f r_w is a focal length of the rear group (G r) in a state where the region in which the image of the front-direction object is formed is in a central region of the observation region, and the substantially lateral-direction object is formed in an annular region thereon and such that simultaneous observation of the front-direction object and the substantially-lateral-direction object is made, and f r_c is a focal length of the rear group (G r) in a state where the region in which the image of the front-direction object is formed is enlarged in the entire observation region, such that enlarged-view observation of the front direction object is made.