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.

A vehicle seat includes a seat cushion and a seatback. The vehicle seat has a plate-shaped pressure-receiving member configured to receive a load from an occupant seated on the vehicle seat and a radio wave sensor fixed to the pressure-receiving member. The radio wave sensor is configured to acquire information about the occupant by emitting radio waves toward the occupant and detecting the radio waves reflected off the occupant. The pressure-receiving member includes a plurality of first ribs that protrudes from a backside of the pressure-receiving member, which is a side facing away from the occupant, in a direction away from the occupant, and a plurality of second ribs that protrudes from the backside in a direction away from the occupant, the plurality of second ribs crossing the plurality of first ribs. The radio wave sensor includes at least one fixing part that is fixed to the pressure-receiving member. The at least one fixing part includes a first fixing part that is fixed to a surrounded area that is surrounded by the plurality of first ribs and the plurality of second ribs on the backside.