To provide a rubber composition giving a smooth surface of the sheet by forming a blend containing silica as a reinforcing agent by the use of a roll and having good tensile characteristics and reduced exothermic tendency, and its manufacturing method.

The composition comprises 20-80 parts weight of (A) a conjugated diene rubber of a Mooney viscosity of 70-200 consisting of 40-100 wt.% of conjugated diene units and 0-60 wt.% of aromatic vinyl monomer units 80-20 parts weight of (B) a conjugated diene rubber of a Mooney viscosity of 20-150 consisting of 40-99.8 wt.% of conjugated diene units, 0-59.8 wt.% of aromatic vinyl monomer units and 0.2-20 wt.% of amino-containing monomer units, in a combined content of (A) and (B) of 100 parts weight, with the viscosity of (B) being at least 10 lower than that of (A).