A seat having a fatigue lessening device functions to repeatedly expand and contract a given part of a seat occupant holding portion of the seat. A fatigue sensor is employed which issues an information signal representative of the fatigue of the occupant. A computer controls the operation of the device at intervals which are derived from the information signal. The fatigue sensor counts the number of movement of the seat occupant and uses the counted number as the factor for representing the degree of the occupant's fatigue.
