A fixing device is provided for a headrest that fixes the headrest at a predetermined position in an unadjustable manner, even after a guide lock is attached or even after a seatback is formed. At least one groove portion is formed by cutting out in a headrest pillar, without inclination in the radial direction. The fixing device has a knob portion, a resin spring portion formed continuously to the knob portion, and an engagement plate arranged in the knob portion, the resin spring portion is formed to extend on one side of the knob portion therefrom, and is provided with a space portion for expanding to surround the headrest pillar. The engagement plate is installed in the knob portion, and is formed in such a manner that part of the engagement plate is exposed in the space portion.