PURPOSE: To reduce the adhesion of mud, in the lugs arranged on a tread part, by forming the outer surface of shoulder parts including the tread part between lugs which adjoin in the circumferential direction in a curved concave surface which has been curved radially inward.

CONSTITUTION: On the outer circumferential surface of the tread part 7 of a tire 1, lugs 8 of a trapezoidal sectional shape extending obliquely at regular intervals in the circumferential direction are arranged. In the above constitution, a curved concave surface 23 is formed on the outer surface of shoulder parts 24 including the tread part 7 between lugs 8 which adjoin in the circumferential direction. And the curved concave surface 23 is formed in a curved concave shape of a radius R whose center of the radius lies in the reverse direction to that of the tire radius, and is formed so as to have a uniform and thin thickness. Thereby mud to be adhered to the boundary 26 between lug feet 18 can be reduced, and also the separation of adhered mud can be expedited. Thus, the function of the lug 8 can be properly secured.