To provide a technique that enables the reduction of fatigue of a vehicle occupant at low cost and without an increase in the number of parts. Without preparing a sensor in addition to a posture changing mechanism, the position of a lumbar vertebra or thoracic vertebra of each vehicle occupant is estimated by controlling the operation of the posture changing mechanism. The posture changing mechanism includes: a back side support mechanism; a first motor capable of shifting a position of the back side support mechanism with respect to a vertical direction; and a second motor capable of shifting the position of the back side support member with respect to a front-back direction. A position of a lumbar vertebra of the vehicle occupant is estimated by: while changing the vertical position of the back side support mechanism by means of the first motor, changing a forward position of the back side support mechanism at each of the different vertical positions thereof by means of the second motor so as to obtain a turnover number value of the second motor achieving a desired resistance; and comparing the turnover number values of the second motor achieving the desired resistance. A position of the thoracic vertebra is determined by calculation based on the position of the lumbar vertebra. The position of the posture changing mechanism is set based on the results of estimation of the individual positions. Thus, a fatigue reduction effect is provided on the vehicle occupant by utilizing the posture changing mechanism.