A single-focus optical system includes a first lens unit having a negative refractive power, a second lens unit having a positive refractive power, and a third lens unit having a positive refractive power. The first lens unit includes a reduction-side lens component closest to the reduction side, and a shape of the reduction-side lens component is a meniscus shape having a convex surface directed toward the enlargement side. The second lens unit includes a cemented lens of a meniscus shape having a convex surface directed toward the reduction side, and a plurality of positive lens components. The cemented lens in the second lens unit includes a negative lens and a positive lens. The third lens unit includes a negative lens component closest to the enlargement side. All air spaces in the first lens unit and in the second lens unit are constant at the time of focusing.