An expandable cage which is used in spinal surgery and which allows adjustment of distance between two vertebrae in which it is placed as preferred. An expandable cage-basically includes at least one body which is located in emptied area between damaged vertebrae during the surgical procedure and serves as a disc; and at least one movable element which is located on the body and is fitted in the opening made in the body by means of the retainers disposed at the sides thereof; which has a toothed surface contacting with the upper vertebra and being attached to it; and the height of which can be adjusted by pushing the protrusion provided in the lower portion thereof according to the intervertebral distance of patient by means of the expansion apparatus.