The monitoring of pore water pressure in fully and partly saturated debris flows was performed at Ohya landslide scar, central Japan. The pore water pressure in some partly saturated flows greatly exceeded the hydrostatic pressure. The depth gradient of the pore water pressure in the lower part of the flow was generally higher than the upper part of the flow. We conclude that excess pore water pressure is present in many debris flow surges and is an important mechanism in debris flow behavior.