We investigate excitation process of the infrasonic waves associated with Merapi-type pyroclastic flows at Unzen volcano, Japan, using data set obtained by a new system that permits recording of geophysical data with accompanying visual events on a single videotape. The accurate correlation of infrasonic waves with video images reveals that: (1) successive impulsive waves and the low-frequency wave (<~0.8Hz) are excited during the collapse of lava blocks falling from the dome, which generates a pyroclastic flow; (2) an impulsive wave is excited by an explosive blowout of the pore-gases which are released by the break-down of the falling blocks; (3) the low-frequency wave is excited by the volumetric change which results from expansion of the pore-gases released by pulverization of the falling blocks. The comparison between the observed and the synthetic low-frequency waves shows that the amount of volume change is about 2 times that of the initial falling blocks. ¿ 2001 American Geophysical Union