Provided is a single-focus optical system which is configured, in order from the object side to the image side, of a first to third lens groups and in which the first lens group and the third lens group are fixed with respect to a predetermined imaging surface, and the second lens group is moved in the optical axis direction to focus, wherein the first lens group comprises at least one positive lens and at least one negative lens, the second lens group comprises at least one positive lens, the third lens group comprises at least one lens having at least one aspheric surface and having a positive optical power at a peripheral portion thereof and 5