Disclosed is a projection optical system, including a first lens group configured to include at least one lens and have a positive refractive power, a second lens group configured to include at least one lens and have a positive refractive power, a third lens group configured to include at least one lens and have a negative refractive power, a fourth lens group configured to include at least one lens configured to have an aspheric surface or a free-form surface, and a concave mirror, wherein the first lens group, the second lens group, the third lens group, the fourth lens group, and the concave mirror are arranged in order and the projection optical system is a non-telecentric optical system.