The present invention relates to a headrest locking device, and more particularly, to a headrest locking device including a case and a lock plate slidably accommodated in the case and inserted in an engaging groove of a stay rod. Here, a horizontal tension rib is located between the lock plate and the case. The horizontal tension rib applies an elastic force to the lock plate in a direction different from a sliding direction of the lock plate, and the lock plate has a panel shape formed by integrating a body inserted into the engaging groove with the horizontal tension rib. A gap between the lock plate and the case is eliminated and a restoring function is embodied at the same time, and manufacturing costs of the lock plate are reduced.