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On September 1, 2004, Mt. Asama in central Japan erupted for the first time in 21 years. Between this moderate eruption and mid-November of the same year, 4 additional moderate eruptions occurred. We installed 8 broadband seismic stations in addition to the short period seismic network around the volcano and succeeded in recording the near-field seismic signals associated with the summit eruptions. The results of the waveform inversions clearly show that the force system exerted at the source region is dominated by vertical single force components. The source depths of the single force are shallower than 200 m from the bottom of the summit crater, and the order of magnitude of the single force is 1010–1011N. The source time history of each vertical single force component consists of two downward forces and one upward force. The initial downward force probably corresponds to the sudden removal of a lid capping the pressurized conduit. The drag force due to viscous magma moving upward in the conduit can explain the upward force. The correlation between the single force amplitudes and the amounts of volcanic deposits emitted from the summit crater are not necessarily positive, suggesting that the amount of deposits remaining within the summit crater may have played an important role in the excitation of the single force.