Disclosed is calcium carbonate treated with a curing active ingredient, which, when compounded in a rubber, enables to provide good workability (handling) to the rubber and maximize advantageous properties of the rubber attributable to calcium carbonate (e.g., slip resistance) without deteriorating the reinforcing property of the rubber. The calcium carbonated treated with a curing active ingredient is produced by mixing a modified calcium carbonate having its surface treated with a fatty acid or a resin acid and silicic acid and having an average primary particle diameter of 0.01 to 0.5 μm with a curing active ingredient which takes a liquid form at ambient temperature so that the content of the curing active ingredient in the finished product becomes 9.1 to 35 wt %.