BACK SUPPORT DEVICE AND VEHICLE SEAT

A back support device, comprising: a frame portion provided at a seat on which a seated person is seated, the frame portion including a locked portion at an upper portion of the frame portion; an upper frame portion fixed to the upper portion of the frame portion and extending in a seat width direction below the locked portion; a lumbar support portion engaged with a lower portion of the frame portion and configured to support a waist of the seated person from a seat rear side; and a pressure receiving portion including, at an upper end portion thereof, a locking portion locked to the locked portion, the pressure receiving portion being supported by the upper frame portion and the lumbar support portion from the seat rear side, and being configured to be deformable by receiving a load from a back of the seated person.