Pressure-induced crystallization from amorphous calcium carbonate (ACC) was observed with X-ray diffraction at room temperature. Time-resolved diffraction patterns revealed that calcite and vaterite simultaneously crystallized from ACC, without transformation from vaterite to calcite against Ostwald's step rule. The n values of calcite in Avrami modeling were slightly larger than that of vaterite, which was consistent with the slightly larger grain size of calcite compared with vaterite. Microtexture of the recovered samples suggested that vaterite and calcite crystallized by noticeably different mechanisms.