We have developed high-speed frequency modulation AFM (FM-AFM) and enabled atomic-resolution imaging in liquid at ~1 s/frame. With this AFM, we have obtained images revealing the transition region is formed along the step edges. Our simulations suggest the transition region be a Ca(OH)2 monolayer formed as an intermediate state. Thus, our understanding of the calcite dissolution in water is much improved. This FM-AFM will be applicable to various studies on solid-liquid interface dynamic processes.