A car seat includes an actuator capable of changing an orientation of a rest surface of a seat back laterally by turning and moving at least a side portion frame that is part of the car seat frontward and rearward, and a controller configured to exercise control over the actuator. The controller includes a run-time posture support unit configured to execute a run-time posture support control under which when a car makes a turn, the actuator is regulated to cause the rest surface of the seat back to be oriented in a direction of the turn. The run-time posture support unit is configured to regulate the actuator in accordance with a placement of the seat in the car, when the run-time posture support unit executes the run-time posture support control.