Projection optical system and projector

A projection optical system that projects, while enlarging, an image displayed on an image display surface, the projection optical system comprising, in order from an enlargement side:

• a first negative refractive power lens group;
• a second negative refractive power lens group;
• a stop;
• a first positive refractive power lens group;
• at least one positive or negative refractive power lens groups disposed between the second negative refractive power lens group, wherein
• the projection optical system causes magnification variation by varying distances between the lens groups,
• the stop is movable during the magnification variation,
• the first positive refractive power lens group is the only lens group that is positioned closer to a reduction side than the stop and is also movable during the magnification variation, and
• the first positive refractive power lens group is separated, by a greatest distance among lens group, into a front group having a negative refractive power and a rear group having a positive refractive power, and conditional formulae (1) and (2) are fulfilled:
• −20<frf/Tr<−5 (1)
• −10<frf/frr<−1 (2)
• where
• frf represents a focal length of the front group,
• frr represents a focal length of the rear group, and
• Tr represents an axial distance between the front group and the rear group.