A tyre comprising a tread portion, wherein the tread portion is provided with a tread ground contacting surface and at least one main groove (3, 4, 5) having an opening at the tread ground contacting surface and extending continuously in a tyre circumferential direction, in a reference state in which the tyre is mounted on a standard rim, inflated to a standard inner pressure, and placed on a flat surface with zero camber angle, when the tread portion of the tyre is loaded with a standard tyre load, the tread ground contacting surface is in contact with the flat surface, the at least one main groove (3, 4, 5) has a bottom surface and two groove wall surfaces extending outwardly in a tyre radial direction from the bottom surface, in a lateral cross section taken perpendicular to a length direction of the at least one main groove (3, 4, 5), at least one of the two groove wall surfaces is provided with an inner groove wall portion connected with the bottom surface and a protruding portion connected with the inner groove wall portion and having a protruding end protruding inwardly in a groove width direction from a groove reference line obtained by extending the inner groove wall portion, characterized in that the protruding portion has an inclined surface portion which is inclined and extends between the protruding end and the tread ground contacting surface, wherein an angle between the inclined surface portion and the tread ground contacting surface is an obtuse angle of 160 degrees or more, and in the reference state, when a tyre load of 1.20 times the standard tyre load is applied to the tread portion, the whole inclined surface portion is in contact with the flat surface. the tread portion is provided with a tread ground contacting surface and at least one main groove (3, 4, 5) having an opening at the tread ground contacting surface and extending continuously in a tyre circumferential direction, in a reference state in which the tyre is mounted on a standard rim, inflated to a standard inner pressure, and placed on a flat surface with zero camber angle, when the tread portion of the tyre is loaded with a standard tyre load, the tread ground contacting surface is in contact with the flat surface, the at least one main groove (3, 4, 5) has a bottom surface and two groove wall surfaces extending outwardly in a tyre radial direction from the bottom surface, in a lateral cross section taken perpendicular to a length direction of the at least one main groove (3, 4, 5), at least one of the two groove wall surfaces is provided with an inner groove wall portion connected with the bottom surface and a protruding portion connected with the inner groove wall portion and having a protruding end protruding inwardly in a groove width direction from a groove reference line obtained by extending the inner groove wall portion, characterized in that the protruding portion has an inclined surface portion which is inclined and extends between the protruding end and the tread ground contacting surface, wherein an angle between the inclined surface portion and the tread ground contacting surface is an obtuse angle of 160 degrees or more, and in the reference state, when a tyre load of 1.20 times the standard tyre load is applied to the tread portion, the whole inclined surface portion is in contact with the flat surface.