Back support device and vehicle seat

A back support device, comprising:

• a first frame portion provided at a seat on which a seated person is seated, the first frame portion including a locked portion at an upper portion of the first frame portion;
• a second frame portion fixed to the upper portion of the first frame portion and extending in a seat width direction below the locked portion;
• a lumbar support portion engaged with a lower portion of the first 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 second 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,
• wherein a protrusion extending in the seat width direction and protruding toward the seat rear side is formed at a position facing the second frame portion at a seat rear side of the pressure receiving portion, the protrusion being formed below the locking portion so as to be spaced apart from the locking portion in a vertical direction, and the protrusion being supported by the second frame portion.