PURPOSE: To make improvements in equality, steering stability, durability and adhesion, by using such a cord as being made up of twisting a high-speed spinning fiber of specified material nylon 66 and an aromatic polyamid fiber together, for a carcass ply.

CONSTITUTION: Fused nylon 66 of more than 205 in relative viscosity is spun out, while such a cord as being made up of twisting at least a piece of nylon yarn below 13% in a size coefficient of variation formed by bundling a lot of high- speed spinning nylon filaments being processed in heat treatment under expansion of the spun yarn cooled and solidfied as extended so as to cause a coefficient of double refraction to become 20×10-3-40×10-3 and at least a piece of aromatic polyamid yarn together. This cord is used for a carcass ply so that equality, durability, steering stability and adhesion are all improvable.