On July 1, 15, and 25, 2024 in the Katsuyamadani River, which flows through the southwestern foothills of Mt. Ibuki in eastern Shiga Prefecture, debris flows occurred three times in a row, causing damage by sediment flooding houses, prefectural roads, and municipal roads in the downstream Ibuki district. These debris flows occurred despite low rainfall that had not occurred in previous decades in the Katusyamadani River. In order to clarify the cause of the repeated occurrence of debris flows due to such not very strong rainfall, I examined the mechanism of the occurrence of these debris flows from field surveys and interpretation of satellite images at different measurement times. As a result, soil erosion occurred due to the impoverished vegetation grazed by Sika Deer (Cervus Nippon) on the southern slope of Mt. Ibuki, and the amount of surface flow and runoff sediment increased, flowing into the depression in the Middle Katuyamadani River Basin through the gullies. It was clarified that the overflowing water from this depression flowed into the Lower Katsuyamadani River, and the increase in the flow rate of the Lower Katsuyamadani River caused erosion of the valley bed and the valley bank, resulting in debris flows. Until now, there have been no reports of debris flows caused by deer eating damage in Japan, causing damage to houses downstream, and this is considered to be the first case in Japan.