A seat for reducing looseness of a seated posture in the entirety of the body. In connection with a load peak position of the pelvis of a seated occupant as detected by a pelvis displacement sensor disposed in the seat cushion of a seat, a control device compares an initial peak position and a peak position after displacement. At the same time, the control device detects a shoulder blade position from load distribution of the chest of the seated occupant, detected by a chest displacement detecting sensor disposed in the upper portion of a seat back, and compares an initial position detected in advance and an after-displacement position. Next, the control device moves the seating position of a seat cushion in a direction to eliminate dislocation between the initial peak position and the after-displacement peak position, and moves the upper portion of the seat back in a direction to eliminate dislocation between the initial position and the after-displacement position of the shoulder blade of the seated occupant.