Step-like tilt changes were observed at intervals of dozens of hours during the caldera formation stage occurred during the 2000 Miyakejima volcano eruption. Our analysis suggests that these tilt steps were caused by the cyclic expansion of a subsurface sill-like magma plumbing system trending from SE to NW of the volcano. In the same period a caldera gradually grew at the summit area to a very large 1.6 km in diameter and 450 m in depth. The source mechanism of these tilt steps is fairly well explained by a two-phase flow instability, called a pressure drop oscillation. This model indicates that the gas-phase capacity in the upper part of the conduit acts as a pump that controls the flux of two-phase magma in the sill-like reservoir. Caldera formation as well as magma uplift and descent in the conduit may reflect temporal changes in the intervals between tilt steps, which are themselves proportional to the volume of this gas capacity.