A headrest height adjustment device includes a base member fixed on a seatback a seat body, a pulley rotatably supported by the base member, the pulley being rotatable in a first direction by unwinding a first end of a rope member wound around the pulley and winding a second end of the rope member in conjunction with sliding movement of the seat body, and the pulley being rotatable in a second direction by winding the first end of a rope member and unwinding the second end of the rope member in conjunction with sliding movement, a rotational arm rotatably supported by the base member and rotated by the pulley by a rotation transmission member, and a conversion member engaged with the rotational arm for operating a headrest in an upward or downward direction by converting rotational movement of the rotational arm into linear movement.