A vehicle seat comprising: a seat frame (4F, 6F, 8F); a seat pad (4P, 6P, 8P); and a planar body configured to support, from a backside, the seat pad (4P, 6P, 8P) placed inside the seat frame (4F, 6F, 8F), the planar body being provided with a high-elongation part (41, 42) more easily extensible than other planar-body part (51, 52, 53, 54, 55), characterized in that: the planar body is a fabric configured such that a plurality of constituent yarns is entangled; the planar body includes a low-density region in which the constituent yarns extending in the same direction are placed thinly, a high-density region in which the constituent yarns extending in the same direction are placed more thickly than the low-density region, and a medium-density region (FM1, FM2, SM1, SM2, SM3, SM4) in which the constituent yarns extending in the same direction are placed more thickly than the low-density region, but more thinly than the high-density region, the medium-density region being provided between the low-density region and the high-density region adjacent to each other; the low-density region is placed in the high-elongation part (41, 42) so that the high-elongation part (41, 42) is easily extensible; the high-density region and the medium-density region (FM1, FM2, SM1, SM2, SM3, SM4) are placed in the other planar-body part (51, 52, 53, 54, 55) so that the other planar-body part (51, 52, 53, 54, 55) is less easily extensible; and the medium-density region (FM1, FM2, SM1, SM2, SM3, SM4) in the other planar-body part (51, 52, 53, 54, 55) is adjacent to the high-elongation part (41, 42). a seat frame (4F, 6F, 8F); a seat pad (4P, 6P, 8P); and a planar body configured to support, from a backside, the seat pad (4P, 6P, 8P) placed inside the seat frame (4F, 6F, 8F), the planar body being provided with a high-elongation part (41, 42) more easily extensible than other planar-body part (51, 52, 53, 54, 55), characterized in that: the planar body is a fabric configured such that a plurality of constituent yarns is entangled; the planar body includes a low-density region in which the constituent yarns extending in the same direction are placed thinly, a high-density region in which the constituent yarns extending in the same direction are placed more thickly than the low-density region, and a medium-density region (FM1, FM2, SM1, SM2, SM3, SM4) in which the constituent yarns extending in the same direction are placed more thickly than the low-density region, but more thinly than the high-density region, the medium-density region being provided between the low-density region and the high-density region adjacent to each other; the low-density region is placed in the high-elongation part (41, 42) so that the high-elongation part (41, 42) is easily extensible; the high-density region and the medium-density region (FM1, FM2, SM1, SM2, SM3, SM4) are placed in the other planar-body part (51, 52, 53, 54, 55) so that the other planar-body part (51, 52, 53, 54, 55) is less easily extensible; and the medium-density region (FM1, FM2, SM1, SM2, SM3, SM4) in the other planar-body part (51, 52, 53, 54, 55) is adjacent to the high-elongation part (41, 42). the planar body is a fabric configured such that a plurality of constituent yarns is entangled; the planar body includes a low-density region in which the constituent yarns extending in the same direction are placed thinly, a high-density region in which the constituent yarns extending in the same direction are placed more thickly than the low-density region, and a medium-density region (FM1, FM2, SM1, SM2, SM3, SM4) in which the constituent yarns extending in the same direction are placed more thickly than the low-density region, but more thinly than the high-density region, the medium-density region being provided between the low-density region and the high-density region adjacent to each other; the low-density region is placed in the high-elongation part (41, 42) so that the high-elongation part (41, 42) is easily extensible; the high-density region and the medium-density region (FM1, FM2, SM1, SM2, SM3, SM4) are placed in the other planar-body part (51, 52, 53, 54, 55) so that the other planar-body part (51, 52, 53, 54, 55) is less easily extensible; and the medium-density region (FM1, FM2, SM1, SM2, SM3, SM4) in the other planar-body part (51, 52, 53, 54, 55) is adjacent to the high-elongation part (41, 42).