A rack shaft supporting structure (15; 15P) comprising: a rack shaft that has a rack that meshes with a pinion, and an axial groove (16; 16P) that is formed in a back face of the rack shaft; a housing which has an accommodation portion, and into which the rack shaft is inserted, the accommodation portion extending in an orthogonal direction orthogonal to an axial direction of the rack shaft; a rack guide (19; 19P) that is accommodated in the accommodation portion, and supports the rack shaft in such a manner that the rack shaft is slidable in the axial direction; a sealing member (21; 21P) that is fixed to an outward opening end that is provided at a side of the accommodation portion, the side being opposite to the rack shaft; an urging member that is interposed between the sealing member (21; 21P) and the rack guide (19; 19P) so as to urge the rack guide (19; 19P) toward the rack shaft; a rack guide supporting member (24; 24P) that has a supporting hole (23; 23P, 23Q) that supports a part of the rack guide (19; 19P) in such a manner that the rack guide (19; 19P) is slidable in the orthogonal direction, the rack guide supporting member (24; 24P) being supported by the housing or the sealing member (21; 21P); and an elastic body (25, 26) that is supported by the rack guide supporting member (24; 24P), is accommodated in the axial groove (16; 16P) of the rack shaft, and is able to press the rack shaft in a radial direction, wherein a bottom face (16a; 16Pa) of the axial groove (16; 16P) of the rack shaft includes a first region (61; 61P) that includes a first axial end (51; 51P) that faces the elastic body in the orthogonal direction when the rack shaft is at one stroke end, a second region (62; 62P) that includes a second axial end (52; 52P) that faces the elastic body in the orthogonal direction when the rack shaft is at the other stroke end, and an intermediate region (63; 63P) that is interposed between the first (61; 61P) and second regions (62; 62P); characterized in that the intermediate region (63; 63P) avoids contacting the elastic body (25, 26), wherein a chevron-shaped protrusion (70, 80) that compresses the elastic body (25, 26) in the orthogonal direction is provided in each of the first (61; 61P) and second regions (62; 62P), and wherein the chevron-shaped protrusion (70, 80) includes a first slope face (71, 81) that is disposed on a side of the intermediate region (63; 63P), and a second slope face (72, 82) that is disposed on a side of a corresponding one of the first (51; 51P) and second axial ends (52; 52P), and that tilts toward a side opposite to a side toward which the first slope face (71, 81) tilts. a rack shaft that has a rack that meshes with a pinion, and an axial groove (16; 16P) that is formed in a back face of the rack shaft; a housing which has an accommodation portion, and into which the rack shaft is inserted, the accommodation portion extending in an orthogonal direction orthogonal to an axial direction of the rack shaft; a rack guide (19; 19P) that is accommodated in the accommodation portion, and supports the rack shaft in such a manner that the rack shaft is slidable in the axial direction; a sealing member (21; 21P) that is fixed to an outward opening end that is provided at a side of the accommodation portion, the side being opposite to the rack shaft; an urging member that is interposed between the sealing member (21; 21P) and the rack guide (19; 19P) so as to urge the rack guide (19; 19P) toward the rack shaft; a rack guide supporting member (24; 24P) that has a supporting hole (23; 23P, 23Q) that supports a part of the rack guide (19; 19P) in such a manner that the rack guide (19; 19P) is slidable in the orthogonal direction, the rack guide supporting member (24; 24P) being supported by the housing or the sealing member (21; 21P); and an elastic body (25, 26) that is supported by the rack guide supporting member (24; 24P), is accommodated in the axial groove (16; 16P) of the rack shaft, and is able to press the rack shaft in a radial direction, wherein a bottom face (16a; 16Pa) of the axial groove (16; 16P) of the rack shaft includes a first region (61; 61P) that includes a first axial end (51; 51P) that faces the elastic body in the orthogonal direction when the rack shaft is at one stroke end, a second region (62; 62P) that includes a second axial end (52; 52P) that faces the elastic body in the orthogonal direction when the rack shaft is at the other stroke end, and an intermediate region (63; 63P) that is interposed between the first (61; 61P) and second regions (62; 62P); characterized in that the intermediate region (63; 63P) avoids contacting the elastic body (25, 26), wherein a chevron-shaped protrusion (70, 80) that compresses the elastic body (25, 26) in the orthogonal direction is provided in each of the first (61; 61P) and second regions (62; 62P), and wherein the chevron-shaped protrusion (70, 80) includes a first slope face (71, 81) that is disposed on a side of the intermediate region (63; 63P), and a second slope face (72, 82) that is disposed on a side of a corresponding one of the first (51; 51P) and second axial ends (52; 52P), and that tilts toward a side opposite to a side toward which the first slope face (71, 81) tilts.