To provide a diene rubber composition suitable e.g. as a vulcanizing molding material of a reinforcing liner layer having a crescent cross-section and placed at the inner side of carcass layers of the right and left sidewalls of a run flat tire, and enabling the improvement in the run flat durability by suppressing the increase of heat-generation without lowering the storage elastic modulus.

The diene rubber composition is produced by compounding (a) 100 pts.wt. of a diene rubber containing 20-40 pts.wt. of natural rubber and 40-80 pts.wt. of a polybutadiene rubber with (b) 30-70 pts.wt. of carbon black, (c) 30-70 pts.wt. of crushed bituminous coal and (d) 5-8 pts.wt. of sulfur.