A lumbar support plate which is supported movably in a front/rear direction in a back frame that supports a headrest, and a biasing member which supports the lumbar support plate, are provided. A clutch mechanism which enables an output member to rotate by a rotating operation of a handle shaft, and an adjustment member which is capable of adjusting the front/rear position of the lumbar support plate in relation with an output member of the clutch mechanism are also provided. Also provided is a linking release mechanism which temporarily releases linkage with the clutch mechanism and permits rearward movement of the lumbar support plate, when a load equal to or greater than a prescribed value is applied to the lumbar support plate.