A rubber composition includes a vulcanization promoter composition which comprises a modified calcium carbonate (A) surface-treated with a fatty acid or a resin acid and a silicic acid, an inorganic filler (B) exhibiting an oil absorption of as high as 50 to 300 ml/100 g, and a vulcanization promoter component (C) liquid at ordinary temperatures, characterized in that the (A):(B) mixing ratio is within the range of 30:70 to 95:5 and the content of (C) is 30 to 80 wt % based on the whole of the promoter composition and precipitated silica are blended into a rubber.