The extender lens apparatus is detachably attached to an image-forming optical system including a first main lens unit disposed closest to an object and a second main lens unit disposed between the first main lens unit and an image plane. The apparatus includes a first extender lens unit to be inserted in a space closer to the first main lens unit than the second main lens unit, and a second extender lens unit to be inserted in a space closer to the image plane than the second main lens unit. The first extender lens unit has a magnification increasing effect. The apparatus is capable of varying a magnification of the image-forming optical system while maintaining good optical performance without greatly increasing the entire length of the image-forming optical system.