A large-scale rock landslide occurred along the Hidakahoronai River during the 2018 Hokkaido Eastern Iburi Earthquake. The ridge with a length of about 1200 m and a width of about 400 m slid about 350 m, hit the opposite slope and stopped, damming the river. This landslide is estimated to be a primary rock glide, because no microtopography of the slope deformation was observed before sliding. For clarifying the topographical geological conditions of large-scale primary rock landslides and the motion image of slid mass, we analyzed the topography before and after the occurrence and estimated the moving velocity based on the transmission line damage record. From the geomorphological analysis, the main block is estimated to translate on the low-angle （about 6 degree） slip surface along bedding plane, and also it can be seen as a spread type （spread ratio 1.7） included separation blocks of the head. From the analysis of the records of the damage of the transmission line tower and the transmission abnormality, the high speed value of about 25 to 60 km / h on average in the first half of the sliding was obtained. From the spread-type landslide and high-speed movement, it is assumed that the friction of the slip surface in the bedrock is significantly reduced. These inferences are consistent with the mechanism of rock landslide occurrence during an earthquake, which is assumed to have slipped due to slip surface liquefaction.