A single-focus optical system includes a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, and a third lens unit having a positive refractive power. The first lens unit includes at least a first sub-unit, a second sub-unit, and a third sub-unit. The first sub-unit includes one of, a one negative lens component and a plurality of negative lens components that are continuous, the second sub-unit includes two meniscus lens components having respective concave surfaces thereof facing mutually, and the third sub-unit includes a plurality of positive lens components disposed closest to the reduction side in the third sub unit. The second lens unit includes one negative lens component. The third lens unit includes negative lens, and also includes a positive lens component closest to the enlargement side. All air spaces in the first lens unit are constant.