An optical system comprising a plurality of lens units, intervals between adjacent lens units being changed during focusing, wherein the plurality of lens units consists of a first lens unit that has positive refractive power and is disposed closest to an object side in the optical system, a middle group (Lm) including at least two lens units, and a final lens unit (Ln) that has negative refractive power and is disposed closest to an image side in the optical system, wherein the middle group (Lm) includes a first focus lens unit (Lf) configured to move during focusing, and a second focus lens unit (Lr) disposed on the image side of the first focus lens unit (Lf) and configured to move during focusing, wherein the middle group (Lm) consists of a second lens unit having negative refractive power, a third lens unit having positive refractive power and a fourth lens unit having negative refractive power, disposed in order from the object side to the image side in the optical system, the second lens unit is the first focus lens unit (Lf), and the fourth lens unit is the second focus lens unit (Lr), or wherein the middle group (Lm) consists of a second lens unit having negative refractive power, a third lens unit having positive refractive power, a fourth lens unit having positive refractive power and a fifth lens unit having positive refractive power, disposed in order from the object side to the image side in the optical system, the second lens unit is the first focus lens unit (Lf), and the fifth lens unit is the second focus lens unit (Lr), wherein an aperture stop is disposed between two lenses included in the middle group (Lm), and wherein the following conditional expressions are satisfied: − 7.00 < f/fn < − 1.78 1.10 < fr/fn < 5.50 4.00 < f / sk < 9.00, where f represents a focal length of the optical system, fn represents a focal length of the final lens unit (Ln), fr represents a focal length of the second focus lens unit (Lr) and sk represents a back focus of the optical system during focusing on an object at infinity. wherein the plurality of lens units consists of a first lens unit that has positive refractive power and is disposed closest to an object side in the optical system, a middle group (Lm) including at least two lens units, and a final lens unit (Ln) that has negative refractive power and is disposed closest to an image side in the optical system, wherein the middle group (Lm) includes a first focus lens unit (Lf) configured to move during focusing, and a second focus lens unit (Lr) disposed on the image side of the first focus lens unit (Lf) and configured to move during focusing, wherein the middle group (Lm) consists of a second lens unit having negative refractive power, a third lens unit having positive refractive power and a fourth lens unit having negative refractive power, disposed in order from the object side to the image side in the optical system, the second lens unit is the first focus lens unit (Lf), and the fourth lens unit is the second focus lens unit (Lr), or wherein the middle group (Lm) consists of a second lens unit having negative refractive power, a third lens unit having positive refractive power, a fourth lens unit having positive refractive power and a fifth lens unit having positive refractive power, disposed in order from the object side to the image side in the optical system, the second lens unit is the first focus lens unit (Lf), and the fifth lens unit is the second focus lens unit (Lr), wherein an aperture stop is disposed between two lenses included in the middle group (Lm), and wherein the following conditional expressions are satisfied: − 7.00 < f/fn < − 1.78 1.10 < fr/fn < 5.50 4.00 < f / sk < 9.00, where f represents a focal length of the optical system, fn represents a focal length of the final lens unit (Ln), fr represents a focal length of the second focus lens unit (Lr) and sk represents a back focus of the optical system during focusing on an object at infinity.