PURPOSE: To prevent the deterioration of the workability in the rim assembly and suppress the generation of the breakage of a bead part in the rim assembly work, by forming a bead core to a rectangular shaped section by using a bead cord and regulating the diameter and the 100% modulus of the topping rubber to each optimum value.

CONSTITUTION: As for a tire which is constitutes so that a bead part is installed at the inner edge in the radial direction of a side wall part which extends inwardly ib the radial direction from both the ends of a tread part, and a bead core 1 is arranged in each bead part, a cord basic body using the hard steel wire having a diameter (d) of 1.2-1.6mm is used in the formation of the bead core 1. A bead cord having the diameter D in 1.2-1.5 times of the diameter (d) of the cord basic body is formed by covering the cord basic body by the topping rubber having a modulus in 100% elongation of 70-85kg/cm2. The bead core 1 having a rectangular shape in the superposition in the radial direction of plural stages of coil layers, on the tire meridian sectional surface including the tire axis, is formed by winding the bead cords in spiral form continuously.