A headrest fore-aft position adjuster comprising: an upper bracket which receives load of an occupant's head and which displaces in a fore-aft direction with respect to a headrest pillar, the upper bracket fixed to an inner cover of a headrest; a lower bracket which is fixed to the headrest pillar; one link and an other link which are each pivotally connected with the lower bracket and the upper bracket and which allows the upper bracket to move along the fore-aft direction of a vehicle seat; and a lock mechanism which locks and unlocks the upper bracket at a plurality of positions along the fore-aft direction, wherein the lock mechanism comprises: a plurality of engaged parts which are provided along a trajectory of a lock pin, which the lock pin is engaged with in a locked state, and which are aligned on a side wall of the one link opposed to the upper bracket along a rotational direction of the one link, a through hole which is provided on a side wall of the upper bracket opposed to the one link at a position on a rotational trajectory of the plurality of engaged parts according to rotation of the one link; and a lock member which comprises: the lock pin which displaces in the fore-aft direction integrally with the upper bracket, and whose tip is pushed into and pulled out from the plurality of engaged parts from a lateral position while the lock pin remains inserted through the through hole, a shaft which is placed in parallel with the lock pin and which is inserted through a shaft insertion hole which laterally penetrates the upper bracket; and a connection part which connects the lock pin with the shaft and which abuts and separates from the side wall of the upper bracket according to push-in and pull-out movement of the lock pin, wherein the shaft is provided with a biasing member which biases the shaft in such a direction that the connection part abuts the side wall of the upper bracket and the lock pin is pushed in the engaged parts. an upper bracket which receives load of an occupant's head and which displaces in a fore-aft direction with respect to a headrest pillar, the upper bracket fixed to an inner cover of a headrest; a lower bracket which is fixed to the headrest pillar; one link and an other link which are each pivotally connected with the lower bracket and the upper bracket and which allows the upper bracket to move along the fore-aft direction of a vehicle seat; and a lock mechanism which locks and unlocks the upper bracket at a plurality of positions along the fore-aft direction, wherein the lock mechanism comprises: a plurality of engaged parts which are provided along a trajectory of a lock pin, which the lock pin is engaged with in a locked state, and which are aligned on a side wall of the one link opposed to the upper bracket along a rotational direction of the one link, a through hole which is provided on a side wall of the upper bracket opposed to the one link at a position on a rotational trajectory of the plurality of engaged parts according to rotation of the one link; and a lock member which comprises: the lock pin which displaces in the fore-aft direction integrally with the upper bracket, and whose tip is pushed into and pulled out from the plurality of engaged parts from a lateral position while the lock pin remains inserted through the through hole, a shaft which is placed in parallel with the lock pin and which is inserted through a shaft insertion hole which laterally penetrates the upper bracket; and a connection part which connects the lock pin with the shaft and which abuts and separates from the side wall of the upper bracket according to push-in and pull-out movement of the lock pin, wherein a plurality of engaged parts which are provided along a trajectory of a lock pin, which the lock pin is engaged with in a locked state, and which are aligned on a side wall of the one link opposed to the upper bracket along a rotational direction of the one link, a through hole which is provided on a side wall of the upper bracket opposed to the one link at a position on a rotational trajectory of the plurality of engaged parts according to rotation of the one link; and a lock member which comprises: the lock pin which displaces in the fore-aft direction integrally with the upper bracket, and whose tip is pushed into and pulled out from the plurality of engaged parts from a lateral position while the lock pin remains inserted through the through hole, a shaft which is placed in parallel with the lock pin and which is inserted through a shaft insertion hole which laterally penetrates the upper bracket; and a connection part which connects the lock pin with the shaft and which abuts and separates from the side wall of the upper bracket according to push-in and pull-out movement of the lock pin, wherein the lock pin which displaces in the fore-aft direction integrally with the upper bracket, and whose tip is pushed into and pulled out from the plurality of engaged parts from a lateral position while the lock pin remains inserted through the through hole, a shaft which is placed in parallel with the lock pin and which is inserted through a shaft insertion hole which laterally penetrates the upper bracket; and a connection part which connects the lock pin with the shaft and which abuts and separates from the side wall of the upper bracket according to push-in and pull-out movement of the lock pin, wherein the shaft is provided with a biasing member which biases the shaft in such a direction that the connection part abuts the side wall of the upper bracket and the lock pin is pushed in the engaged parts.