Vehicle seat

1. A vehicle seat comprising a seat cushion, a seat back, and a headrest, the seat back including a seat back frame constituting a frame of the seat back, and a seat back pad covering the seat back frame, and the vehicle seat being configured to allow an upper body of an occupant to sink into the seat back when a backward movement load acts on the seat back from the occupant's upper body,

wherein the seat back pad comprising a center portion configured to face a back region of the occupant's upper body, and right and left side portions provided at right and left sides of the center portion and each configured to protrude frontward farther than the center portion, said seat back pad having through-openings or recessed portions at positions where regions between the center portion and the right and left side portions overlap the support members,

wherein each of the right and left side portions has an upper side portion corresponding to a height of a chest region of the occupant and a lower side portion corresponding to a height of a lumbar region of the occupant, which are configured such that when they receive the backward movement load, the upper side portion is more easily deformable than the lower side portion,

wherein the vehicle seat further comprises support members provided in the seat back and configured to support the upper side portions from inside the seat back and to be movable in a backward direction, and

wherein the support members are disposed frontward of right and left side frames of the seat back frame with a gap provided between each of the side frames and each of the support members.

2. The vehicle seat according to claim 1 , wherein each support member has a relief portion for preventing interference with the side frame, at a position facing the side frame.

3. The vehicle seat according to claim 1 , wherein the right and left side frames comprise right and left vertical pipe portions extending in an upper-lower direction, and

wherein each of the support members curves along a peripheral surface of a corresponding vertical pipe portion to provide a recessed portion for preventing interference with the side frame at a position facing the side frame.

4. The vehicle seat according to claim 1 , wherein the right and left side frames comprise right and left vertical pipe portions extending in an upper-lower direction at positions corresponding to a height position of the support members, and

wherein each of the support members has a relief portion for preventing interference with a corresponding vertical pipe portion.

5. The vehicle seat according to claim 1 , wherein front surfaces of the support members are disposed in contact with the seat back pad.

6. The vehicle seat according to claim 5 , wherein the support members are configured to urge the upper side portions in a frontward direction.

7. The vehicle seat according to claim 1 , further comprising a pressure-receiving member disposed in the seat back at a position facing the center portion and connected to the right and left side frames of the seat back frame via a connecting member so as to allow the pressure-receiving member to move backward,

wherein the support members are integrally formed with the pressure-receiving member such that they extend outward in a right-left direction from upper portions of right and left ends of the pressure-receiving member.