A run flat tire comprising: a carcass extending between a pair of bead cores of bead portions via a tread portion and a pair of sidewall portions, a belt layer formed by a plurality of belt plies (7A, 7B) arranged outside the carcass in a tire radial direction within the tread portion, and a pair of side reinforcing rubber layers each arranged inside the carcass in each of the sidewall portions and having a substantially crescent cross-sectional shape, wherein each of the side reinforcing rubber layers comprises a first rubber portion arranged on a side of the tread portion and a second rubber portion connected with the first rubber portion so as to have a boundary surface therebetween, extending from the boundary surface toward the adjacent bead portion, and having a modulus at 2% elongation measured at 70 degrees Celsius according to the test method described in Japanese Industrial Standard JIS K6251, the modulus in the second rubber portion being larger than in the first rubber portion and constant between the boundary surface and the adjacent bead portion, in a tire meridian section passing through a tire rotational axis and in each of the side reinforcing rubber layers, a first point where the boundary surface intersects with an outer surface of the side reinforcing rubber layer in a tire axial direction is arranged on a side of the adjacent bead portion in the tire radial direction of a second point of the outer surface which is a closest point to an outer end of the belt layer in the tire axial direction, a third point where the boundary surface intersects with an axially inner surface of the side reinforcing rubber layer is arranged on a side of the adjacent bead portion in the tire radial direction of the first point and a fourth point which is a closest point of the axially inner surface to the outer end, and a distance L1 along the outer surface between an axially inner end of the first rubber portion and the second point is larger than a distance L2 along the outer surface between the second point and the first point, characterized in that in a standard state in which the run flat tire is mounted on a standard rim, inflated to a standard pressure, and loaded with no tire load, a distance L8 in the tire radial direction between the bead base line and the first point is in a range of from 75% to 80% of the tire section height and a distance L9 in the tire radial direction between a bead base line and the third point is in a range of from 70% to 75% of a tire section height, and in that each of the bead portions is provided with an outer bead apex rubber having a larger modulus than the second rubber portion and arranged axially outside the carcass so as to extend radially outwardly from the bead core in a tapered manner. a carcass extending between a pair of bead cores of bead portions via a tread portion and a pair of sidewall portions, a belt layer formed by a plurality of belt plies (7A, 7B) arranged outside the carcass in a tire radial direction within the tread portion, and a pair of side reinforcing rubber layers each arranged inside the carcass in each of the sidewall portions and having a substantially crescent cross-sectional shape, wherein each of the side reinforcing rubber layers comprises a first rubber portion arranged on a side of the tread portion and a second rubber portion connected with the first rubber portion so as to have a boundary surface therebetween, extending from the boundary surface toward the adjacent bead portion, and having a modulus at 2% elongation measured at 70 degrees Celsius according to the test method described in Japanese Industrial Standard JIS K6251, the modulus in the second rubber portion being larger than in the first rubber portion and constant between the boundary surface and the adjacent bead portion, in a tire meridian section passing through a tire rotational axis and in each of the side reinforcing rubber layers, a first point where the boundary surface intersects with an outer surface of the side reinforcing rubber layer in a tire axial direction is arranged on a side of the adjacent bead portion in the tire radial direction of a second point of the outer surface which is a closest point to an outer end of the belt layer in the tire axial direction, a third point where the boundary surface intersects with an axially inner surface of the side reinforcing rubber layer is arranged on a side of the adjacent bead portion in the tire radial direction of the first point and a fourth point which is a closest point of the axially inner surface to the outer end, and a distance L1 along the outer surface between an axially inner end of the first rubber portion and the second point is larger than a distance L2 along the outer surface between the second point and the first point, characterized in that in a standard state in which the run flat tire is mounted on a standard rim, inflated to a standard pressure, and loaded with no tire load, a distance L8 in the tire radial direction between the bead base line and the first point is in a range of from 75% to 80% of the tire section height and a distance L9 in the tire radial direction between a bead base line and the third point is in a range of from 70% to 75% of a tire section height, and in that each of the bead portions is provided with an outer bead apex rubber having a larger modulus than the second rubber portion and arranged axially outside the carcass so as to extend radially outwardly from the bead core in a tapered manner.