PURPOSE: To improve operational stability with buckling eliminated by forming a tread surface with three circular arcs, having given relations of radiuses of curvature each other, over from a tread center to a shoulder part in a tire meridian direction cross section.

CONSTITUTION: A tread surface of a tire meridian direction cross section is formed with circular arcs having radiuses of curvature of R1, R2, and R3 respectively, and a distance from a tread center TC to the intersection point of curved surfaces of R1 and R2, a distance between intersection points of curved surfaces of R1 and R2 also R2 and R3, and a distance from the intersection point of curved surfaces of R2 and R3 to a shoulder part are named as K1, K2, and K3 respectively. Here, respective relations, when a tread extension width TDW is made 200 mm or more, are made as follows: R1=11.5×TDW+(-1800--1950); R2=3×TDW+(-250--400); R3=0.5×TDW+(20--20); K1, and K2, and K3 are 45-55%, 25-35%, and 17-23% of TDW/2; and R1/R2≤5, R2/R3≤5. Consequently buckling is eliminated to improve operational stability.