This report describes a newly observed phenomenon: pressure-induced crystallization from amorphous calcium carbonate (ACC). Synthetic ACC samples were pressurized up to 800 MPa at room temperature. Then crystallization of vaterite and calcite was observed from X-ray diffraction patterns. The crystallization pressure depends on the H2O contents of ACCs. The ACC samples with high-H2O content (21 wt %), middle-H2O content (17 wt %), and low-H,0 content (10 wt %) underwent crystallization at pressures higher than 240 MPa, 400 MPa, and 640 MPa, respectively. These results indicate that H2O in the ACC serves an important role in the crystallization process and that we should treat ACC carefully in preparation for analyses such as infrared spectroscopy to obtain the intrinsic information related to amorphous materials.