A thin groove 4 that continuously extends in the tread circumferential direction is provided in a shoulder rib 3. An annular groove 8 continuous in the circumferential direction is provided in a groove wall 4a on the tread equatorial surface side of the thin groove 4 to be contiguous to the groove bottom, in a cross section taken in the width direction of the tread. The shape of the annular groove in a cross section taken in the width direction of the tread is formed such that a corner portion contiguous to the groove wall on the tread equatorial surface side is rounded to a curvature radius R1, a corner portion of the innermost portion of the annular groove that is in the outer side portion in the tire radial direction is rounded to a curvature radius R2, a corner portion of the innermost portion of the annular groove that is in the inner side portion in the tire radial direction is rounded to a curvature radius R3, and a corner portion contiguous to a groove wall 4b on the tread end side is rounded to a curvature radius R4. According to the present invention, it is possible to improve the stone trapping resistance, tear resistance of defense grooves, and groove bottom crack resistance.