Small earthquakes occurred inside the flowing magma during the 1991--1995 activities of Unzen Volcano. This paradoxical phenomenon can be explained when we consider that even a flowing magma fails brittly under the condition that the product of viscosity and strain rate of magma exceeds a certain threshold value. The estimated strain rate of magma in the conduit leads us to the conclusion the brittle failure is expected when the viscosity is over 1011.5 Pa s. Laboratory measurements of samples show that this viscosity range can be attained when the temperature is under 850 ¿C in dry condition, and 800 ¿C with 0.6 wt.% water. Considering the temperature from Fe-Ti oxide geothermometer (780--880 ¿C) and the gas temperature (800 ¿C), the brittle failure condition could be satisfied inside the conduit. The melt failure by shear flow could be the source of the earthquakes occurred at the summit area. ¿ 1999 American Geophysical Union