PURPOSE: To promote a reaction by providing a sufficient temperature, and to make modified calcium carbonate with high-quality, by setting a heating means around a reactor made of Al to modify calcium carbonate with hydrogen fluoride.

CONSTITUTION: Heavy calcium carbonate is fed from the feed opening 5 to the reactor 1 made of Al with stirring by the agitating blade 2 made of Al, etc. The reactor 1 is heated by the hot air 9→10 heated by the furnace 8, and the raw material is heated. Preheated hydrogen fluoride is then introduced from the inlet 3 to it to carry out a reaction. The feed of hydrogen fluoride is stopped, dehumidified air is introduced, and the remaining hydrogen fluoride is replaced with the air. The discharge outlet 6 is opened with continuing the stirring, and calcium carbonate with its surface modified with hydrogen fluoride is taken out. The device will not corrode after the reaction, and modified calcium carbonate having improved flow properties free from impurity can be formed.