The authors studied the relations between yearly and monthly numbers of occurrence of landslides and meteorological data especially on snow cover in Niigata Prefecture, which is well known as one region of higher snow cover and much landslides in Japan, and pointed out that the occurrences are concentrated in the snowmelt season. The continuous observation records of pore water pressure and height of snow cover, carried out by the authors at Ono landslide area, lead to the conclusion that the cause is continuous infiltrations of snowmelt water which increases pore water pressure near the slip surface dring the snowmelt periods.