The present invention relates to a spinal fusion cage which is inserted between vertebral bodies with the lowest height and is height-adjustable in the inserted state, whereby cages having heights within a predetermined range can be replaced by a single cage. Therefore, manufacturers can reduce product groups that need to be produced, and can also reduce product stock. Further, in contrast to the conventional cages having predetermined heights at regular intervals, the height of the inventive cage can be linearly adjusted according to the distance between the vertebral bodies of a patient, and thus a surgery can be performed using the cage adjusted to an optimum height according to the patient's condition.

A bone fusion device includes: a shaft including a porous section and a threaded proximal section configured for facilitating a compression; and a head threadably connected to the threaded proximal section, the head including a plurality of teeth configured for engaging with a bone, the bone fusion device being formed as a fastener.