I numerically calculate upward migration of vesiculated magma in an elastic volcanic conduit, taking into account the bubble growth in melt, elastic stress from the surrounding medium, and degassing from the magma. The results show that when magma containing gas bubbles reaches the ground surface without degassing, the ground deformation accelerates just before the eruption in response to volumetric expansion of the magma. On the other hand, the rate of change in ground deformation is almost constant when the magma degasses as it ascends. Similar characteristics have been observed in ground deformation recorded before explosive eruptions and lava extrusion at several active volcanoes. These consistencies strongly suggest that geodetic measurements, which have been mainly used to date to estimate the eruption time and the locations of magmatic intrusions, provide a basis for predicting the volcanic explosivity and monitoring bubble growth and degassing processes in the conduit.