A detailed description is presented of hazards that caused the 2017 Akatani River flood disaster, which affected a large area from mountains through lowlands. Hazards are analyzed focusing on sediment transport, fluvial topography, and characteristics of house damage. Based on field surveys and aerial photograph interpretation, lowlands are classified into alluvial fans, colluvial slopes, terraces (I, II, and III), valley plains (I and II) and former channels, and fill and cut land. A number of slope failures occurred on mountain slopes of more than 30 degrees, and debris flows reached areas with slopes of 4 to 8 degrees. Alluvial fans formed in tributary valley mouths overlaying former alluvial fan surfaces, indicating recurrences of debris flows. Sediment and driftwood carried by debris flows were transported further downstream by floodwater, and heavy inundation was observable in areas downstream from debris flow deposits. This inundation mainly occurred in valley plains I and II, and was accelerated by channel deformation caused by sediment accumulating with driftwood. Calculations reveal that the sediment transport capacity of the Otoishi River was higher than that of the upper Akatani River; thus, sediment and driftwood from the Otoishi River were actively transported downstream and flooded the lower Akatani River. Through all of these processes, longitudinal sediment fining detected over the lowlands was notable, and only suspended loads flooded the lowest portion of the basin. An investigation of house damage types and their locations shows that more than 53% of houses in the study area were damaged by hazards, consisting of 36% by the flood and 17% by slope failures and debris flows. The investigation also finds a close relationship between the number of damaged houses and topographic surfaces; most of the houses damaged by the flood were located in valley plains, and most of the houses damaged by slope failures and debris flows were observed in alluvial fans. In addition, most of the flood-affected houses were located quite close to main channels, within 10 to 30 m from the channels and 2 to 5 m above the water surface.