A vehicle seat (10; 80; 90; 100) comprising: a seat cushion configured to support buttocks of an occupant; a seat back configured to support a back of the occupant, the seat back including a lumbar spine support configured to support a lumbar spine of the occupant; an elastic force adjustment unit configured to adjust elastic force of the lumbar spine support in a longitudinal direction; a controller configured to operate the elastic force adjustment unit; a unit having a sensor configured to detect a condition of a vehicle when it is travelling and being located in the elastic force adjustment unit, wherein: the controller is configured to cause the elastic force adjustment unit to make the elastic force larger in a case where a detected oscillation of a vehicle by the sensor is above a threshold, than that in a case where oscillation of the vehicle is below the threshold, characterized in that the elastic force adjustment unit is further configured to: adjust, in addition to the elastic force of the lumbar spine support, the elastic force of a sacrum support configured to support a sacrum located below the lumbar spine of the occupant, and elastic force of a thoracic spine support configured to support a thoracic spine located above the lumbar spine of the occupant in the seat back, respectively; and make the elastic force of the lumbar spine support larger than the elastic force of each of the sacrum support and the thoracic spine support, when oscillation of the vehicle is above the threshold. a seat cushion configured to support buttocks of an occupant; a seat back configured to support a back of the occupant, the seat back including a lumbar spine support configured to support a lumbar spine of the occupant; an elastic force adjustment unit configured to adjust elastic force of the lumbar spine support in a longitudinal direction; a controller configured to operate the elastic force adjustment unit; a unit having a sensor configured to detect a condition of a vehicle when it is travelling and being located in the elastic force adjustment unit, wherein: the controller is configured to cause the elastic force adjustment unit to make the elastic force larger in a case where a detected oscillation of a vehicle by the sensor is above a threshold, than that in a case where oscillation of the vehicle is below the threshold, characterized in that the elastic force adjustment unit is further configured to: adjust, in addition to the elastic force of the lumbar spine support, the elastic force of a sacrum support configured to support a sacrum located below the lumbar spine of the occupant, and elastic force of a thoracic spine support configured to support a thoracic spine located above the lumbar spine of the occupant in the seat back, respectively; and make the elastic force of the lumbar spine support larger than the elastic force of each of the sacrum support and the thoracic spine support, when oscillation of the vehicle is above the threshold. the controller is configured to cause the elastic force adjustment unit to make the elastic force larger in a case where a detected oscillation of a vehicle by the sensor is above a threshold, than that in a case where oscillation of the vehicle is below the threshold, characterized in that the elastic force adjustment unit is further configured to: adjust, in addition to the elastic force of the lumbar spine support, the elastic force of a sacrum support configured to support a sacrum located below the lumbar spine of the occupant, and elastic force of a thoracic spine support configured to support a thoracic spine located above the lumbar spine of the occupant in the seat back, respectively; and make the elastic force of the lumbar spine support larger than the elastic force of each of the sacrum support and the thoracic spine support, when oscillation of the vehicle is above the threshold. adjust, in addition to the elastic force of the lumbar spine support, the elastic force of a sacrum support configured to support a sacrum located below the lumbar spine of the occupant, and elastic force of a thoracic spine support configured to support a thoracic spine located above the lumbar spine of the occupant in the seat back, respectively; and make the elastic force of the lumbar spine support larger than the elastic force of each of the sacrum support and the thoracic spine support, when oscillation of the vehicle is above the threshold.