A seat lifter includes: an inner link for changing a height position of an inner lower arm with respect to a floor of a vehicle; an outer link for changing a height position of an outer lower area with respect to the floor; a coupling rod that couples the inner link to the outer link; a planetary gear mechanism capable of causing the coupling rod to rotate relative to the inner lower arm and the outer lower arm; a drive unit capable of driving the planetary gear mechanism; and a power transmission rod that transmits power of the drive unit to the planetary gear mechanism. The planetary gear mechanism is disposed between the inner lower arm and the outer lower arm. The drive unit is disposed outside the inner lower arm in the width direction or outside the outer lower arm in the width direction.