Obtaining a sharp image of magma supply path through dense geophysical observations is important for forecasting time and magnitude of hazardous future eruptions. Here we reveal a clear magma plumbing system using dense seismic and geodetic networks around Mt. Asama, central Japan. Magma intrusions occurred several times beneath the western flank of Mt. Asama, forming a WNW-ESE directed zone with 1 km below sea level. The eastern end of this zone connects a narrow vertical pathway extending right under the summit crater, which erupted in 2004. Monitoring magmatic activity with a well-designed observational network is vital to mitigate future volcano hazards.