A lumbar support device of a seat for a vehicle surrounds and supports the lumbar region of a passenger. In the lumbar support device, a mounting bracket is fastened to a seat back frame. A support panel is coupled to the mounting bracket through link units so as to be movable ahead of the seat back frame. Side support units are coupled to respective opposite ends of the support panel so as to be rotatable. A connection wire is connected between each of the side support units and the corresponding link unit of the support panel such that when the support panel moves forwards, the side support units surround side portions of the lumbar region of the passenger.