PURPOSE: To improve straight running stability and durability at running under low inflation pressure by a structure wherein the mean value of and difference between the respective angles of the reinforcement cords of an upper and a lower carcass cord layers are rendered to be specified values and yet the number of single strands per unit length of the lower layer is rendered to be specified times as many as that of the upper layer.

CONSTITUTION: The means value β of and the difference (α1-α2) between the angles α1 and α2 of respective reinforcement cords are taken in the range of 96W108° and 10W40° respectively. Within the range as mentioned above, the plys-tear caused by the torsional deformation of belt reinforcement layers 5 can be relieved, resulting in improving the straight running property of the tyre. Furthermore, the total denier per 50m/m of the lower carcass cord layer 4d is made 1.1W1.5 times as many as that of the upper carcass cord layer 4u. Consequently, the difference between the stretch applied to the reinforcement cords in the upper carcass cord layer 4u and that of the lower carcass cord layer 4d during molding and vulcanizing processes can be made smaller, resulting in obtaining the excellent durability at running even under low inflation pressure.