Our field observation and time-series analysis inferred from scientific papers and reports, video films, photographs, and eyewitnesses concerning the Sumikawa landslide, indicate: 1) hydrothermal explosions occured at least three times at the Sumikawa Spa; 2) co-erupted hydrothermal explosions occured at least two times at the Akagawa Spa (1.1 km north of the Sumikawa Spa); and 3) the mass-wasting process of the Sumikawa landslide was affected by the hydrothermal explosions at the Sumikawa Spa. The present scenario for the Sumikawa landslide and the Sumikawa Spa's hydrothermal explosions is summarized as follows: 1) the sliding movement of the Sumikawa landslide was accelerated by the surface propagation of shock-wave due to the hydrothermal explosions; and 2) the downslope movement of landslide deposits as debris avalanche was caused by the blast wave of the hydrothermal explosions.