Seat adjustment device

Provided is a seat adjustment device which is capable of suppressing wobbling of first and second plates and a planetary gear. The seat adjustment device (10) comprises: a first plate (1) provided with a first gear section (12); a second plate (2) provided with a second gear section (22); three planetary gears (3) each meshed with the first gear section (12) and the second gear section (22); a planetary gear holding member (5) holding the planetary gears (3) movably in a radial direction of the first and second gear sections (12, 22); a guide member (6) for guiding the planetary gears radially outwardly; and a locking member (7) for lockingly coupling together the planetary gear holding member (5) and the guide member (6), wherein the guide member (6) is configured, when it is rotated in a specific direction with respect to the planetary gear holding member (5), to guide the planetary gears (3) radially outwardly.