A tire comprising: a tread portion provided in its tread surface with a circumferential groove, the circumferential groove extending continuously in the tire circumferential direction, and having a groove bottom, an inside groove side wall extending radially outwardly from the groove bottom and intended to be positioned on the inside of a vehicle when the tire is mounted thereon, and an outside groove side wall extending radially outwardly from the groove bottom and intended to be positioned on the outside of the vehicle, wherein in the cross section of the circumferential groove perpendicular to the longitudinal direction thereof, the inside groove side wall comprises a lower side wall portion extending radially outwardly from the groove bottom, and a protruding portion extending radially outwardly from the lower side wall portion to have a radially outer edge, while protruding from a reference line defined by a radially outwardly extended line from the lower side wall portion, and the protruding portion comprising a sloped portion extending obliquely from the radially outer edge to the tread surface, and the lower side wall portion extends obliquely from an arc portion of the groove bottom at a constant inclination angle (θi), characterized in that an intersection where the sloped portion intersects with the tread surface is located on the outer side of the reference line. a tread portion provided in its tread surface with a circumferential groove, the circumferential groove extending continuously in the tire circumferential direction, and having a groove bottom, an inside groove side wall extending radially outwardly from the groove bottom and intended to be positioned on the inside of a vehicle when the tire is mounted thereon, and an outside groove side wall extending radially outwardly from the groove bottom and intended to be positioned on the outside of the vehicle, wherein in the cross section of the circumferential groove perpendicular to the longitudinal direction thereof, the inside groove side wall comprises a lower side wall portion extending radially outwardly from the groove bottom, and a protruding portion extending radially outwardly from the lower side wall portion to have a radially outer edge, while protruding from a reference line defined by a radially outwardly extended line from the lower side wall portion, and the protruding portion comprising a sloped portion extending obliquely from the radially outer edge to the tread surface, and a lower side wall portion extending radially outwardly from the groove bottom, and a protruding portion extending radially outwardly from the lower side wall portion to have a radially outer edge, while protruding from a reference line defined by a radially outwardly extended line from the lower side wall portion, and the protruding portion comprising a sloped portion extending obliquely from the radially outer edge to the tread surface, and the lower side wall portion extends obliquely from an arc portion of the groove bottom at a constant inclination angle (θi), characterized in that an intersection where the sloped portion intersects with the tread surface is located on the outer side of the reference line.