A seat lifter smoothly lifts and lowers right and left seat frames relative to respective base members so that a seat can be lifted and lowered smoothly. The seat lifter has first to fourth links (4a to 4d), and a coupling (6) that couples the first link (4a) and the second link (4b), and has a first sun gear (62) and a second sun gear (162). The first link (4a) has a first link body (41) that is coupled interlockably to a first planetary gear group (40) with a plurality of first planetary gears (44), and a planetary-gear operating section (5). The second link (4b) has a second link body (141) that is coupled interlockably to a second planetary gear group (140) with a plurality of second planetary gears (144).