PURPOSE: To provide a seat device enabling a driver to have a stable sitting feeling with no burden on his spine so as to get hardly tired even in the case of performing driving operation on the seat for many hours.

CONSTITUTION: Interlocked with the displacement of a center back part 12 in the direction of withdrawing relatively backward from the back face position by the driving means 35 of the middle back adjusting part 33 of a thorax adjusting mechanism, an upper back part 11 is displaced in such a way as to push the back of a driver relatively forward by an upper back adjusting part 31, and a lower back part 13 is advanced by a bottom adjusting part 37 so as to hold the bottom of the driver from behind.