From a temporal deployment of a broadband seismic network at Usu volcano, Japan, we observed long-period (12 sec) volcanic tremors during the first few weeks of the eruptive activity which started in the end of March, 2000. The source of these long period tremors are located relatively deep at a depth of 5 km, and their amplitude variation well correlates with the uplift rate of the eruption area. We thus attribute these long period tremors to the flow induced vibration of a magma chamber and its outlet located around the source region of the long period tremors. This may be the first seismological detection of long-period (>10 sec) vibrations of a deep magma plumbing system.