We analyze seismic waves that are repeatedly excited by small explosions at the summit of Stromboli volcano, Italy, to understand the source process of volcanic explosions. At the end of September 2016, we deployed five broadband seismometers at locations only 100-300 m away from the active craters. Conducting moment tensor inversion to the entire seismic signals at 0.05-0.2 Hz band, we locate the source centroids at a depth of 170 m and 150-200 m west/southwest of the crater where acoustic waves are excited. On the contrary, the source centroids of seismic waves at 0.2-0.5 Hz and 0.5-1.0 Hz bands, which are excited almost at the same time of explosion onset, are located close to the crater. We further conduct semblance analyses to the seismic signals from about 30 s before to the end of explosion. The results show that a small preceding phase about 10-20 s before the onset of each explosion, which is first detected by our very-close observation, is radiated from a depth of 170 m beneath the west of the craters. Then, the source moves about 50 m toward the active crater area just before an explosion. During the explosion, however, the source moves back to the location where the small preceding phase is excited. These spatio-temporal changes estimated by the two source location analyses reflect the lateral migration of magma and gas bubbles, which may be a large slug as presented in many previous studies, in the shallow magma reservoir beneath the crater area.