Extender lens unit, zoom lens, and image pickup apparatus

An extender lens unit to be inserted into and removed from an optical path in a relay lens unit included in a zoom lens consists of a front unit having a positive refractive power, a middle unit having a negative refractive power, and a rear unit having a negative refractive power. The extender lens unit includes three negative lenses. The middle unit includes a negative lens. An average refractive index of positive lenses included in the extender lens unit, an average refractive index of negative lenses included in the extender lens unit, a focal length of the negative lens included in the middle unit, and a focal length of the middle unit are appropriately set.