A tire comprising a tread portion , wherein the tread portion has four main grooves extending continuously linearly along a tire circumferential direction, each main groove includes a first portion that extends in a depth direction of the main groove so as to have an almost constant groove width, and a second portion that has a groove width which increases from the first portion toward a groove bottom, on a transverse cross-section orthogonal to a length direction of the main groove , and one groove wall of the main groove and the other groove wall of the main groove are line-symmetric with respect to a center line of the main groove in a tire axial direction, on the transverse cross-section, wherein a groove width (Wa) of each main groove , which is a length between groove edges that appear on a tread surface of the tread portion , is 3.0% to 6.0% of a tread width (TW), the tread width (TW) being a distance, in the tire axial direction, from one of tread ends (Te) to the other of the tread ends (Te) in the normal state, wherein a tread end (Te) is a ground contact position at the outermost side in the tire axial direction in a state where a normal load is applied to the tire in the normal state and the tire is brought into contact with a plane at a camber angle of 0°, the normal state being a state in which the tire is mounted to a normal rim and is inflated with air to a normal internal pressure, and no load is applied to the tire , and a depth (d2) from a tread surface of the tread portion to the boundary between the first portion and the second portion is 0.40 to 0.50 times a maximum depth (d1) of the main grooves . a tread portion , wherein the tread portion has four main grooves extending continuously linearly along a tire circumferential direction, each main groove includes a first portion that extends in a depth direction of the main groove so as to have an almost constant groove width, and a second portion that has a groove width which increases from the first portion toward a groove bottom, on a transverse cross-section orthogonal to a length direction of the main groove , and one groove wall of the main groove and the other groove wall of the main groove are line-symmetric with respect to a center line of the main groove in a tire axial direction, on the transverse cross-section, wherein a groove width (Wa) of each main groove , which is a length between groove edges that appear on a tread surface of the tread portion , is 3.0% to 6.0% of a tread width (TW), the tread width (TW) being a distance, in the tire axial direction, from one of tread ends (Te) to the other of the tread ends (Te) in the normal state, wherein a tread end (Te) is a ground contact position at the outermost side in the tire axial direction in a state where a normal load is applied to the tire in the normal state and the tire is brought into contact with a plane at a camber angle of 0°, the normal state being a state in which the tire is mounted to a normal rim and is inflated with air to a normal internal pressure, and no load is applied to the tire , and a depth (d2) from a tread surface of the tread portion to the boundary between the first portion and the second portion is 0.40 to 0.50 times a maximum depth (d1) of the main grooves . <EOS>