Rubber composition and pneumatic tire using same

A peroxide-crosslinkable rubber composition having a run flat property comprising:

• (A) more than 60 parts by weight but not more than 95 parts by weight of a rubber component comprising (i) a natural rubber (NR) and (ii) a polybutadiene rubber (BR), and (iii) optionally, at least one rubber selected from the group consisting of a styrene-butadiene copolymer rubber (SBR), an ethylene-propylene rubber (EPM), an ethylene-propylene-diene terpolymer (EPDM), a silicone rubber and a fluororubber,
• (B) at least 5 parts by weight but less than 40 parts by weight of a polar polymer, which comprises 100 parts by weight of an ethylenically unsaturated nitrile-conjugated diene-based high saturation copolymer rubber having a conjugated diene unit content of 30 wt % or less and 20 to 120 parts by weight of a metal salt of an ethylenically unsaturated carboxylic acid blended therein, and
• (C) carbon black having a nitrogen specific surface area (N2SA) of not more than 70 m2/g in an amount so that the total weight of the components (B) and (C) becomes 20 to 70 parts by weight, based upon 100 parts by weight of the total amount of the components (A) and (B).