A powdery white vulcanization promoter composition which is excellent in storage stability, the resistance of blends thereof with rubbers or the like to heat build-up, and design; and rubber compositions containing the same. Specifically, 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/100g, 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.