A projection optical system includes from the enlargement side a first lens unit,
a mirror which is an optical path bending member, a second lens unit, and a third
lens unit. The projection optical system is designed so that a condition (1), 5.5T12/FL12.0,
is fulfilled. Here, T12 is the air equivalent distance between the first
lens unit and the second lens unit, and FL is the overall focal length of the projection
optical system. By fulfilling the condition (1), it is unnecessary that the diameter
of the first lens unit be extremely large. It is also unnecessary to change the
lens shape to prevent interference between the first lens unit and the second lens
units, so that an increase in the cost of the projection optical system can be suppressed.