Abstract: Disclosed is a headrest fore-aft position adjuster including a displaceable body which receives load of an occupant's head and which displaces in a fore-aft direction with respect to a headrest pillar and a lock mechanism which locks and unlocks the displaceable body at a plurality of positions along the fore-aft direction, and the lock mechanism includes an engaging part which displaces in the fore-aft direction integrally with the displaceable body, a plurality of engaged part which is provided along a trajectory of the engaging part and which the engaging part is engaged with in a locked state and the engaging part is configured to engage with the engaged parts from a lateral position.