A tread pattern is provided with: shoulder lug grooves open at ground contact edges; center lug grooves having two ends; circumferential primary grooves formed in a wave-like shape by alternately connecting ends of the center lug grooves and the inner ends in the tire width direction of the shoulder lug grooves; and center blocks defined by the center lug grooves and the pair of circumferential primary grooves. The belt portion of the tire includes first cross belt layers and second cross belt layers. The ratio of the width WB of the center blocks in the tire width direction to the belt width W1 of the belt layer having the shortest width in the tire width direction among the first cross belt layers is in the range from 0.6 to 0.9.