The January 23, 2018 eruption at Kusatsu-Shirane Volcano was a small phreatic eruption with limited observed precursors. As the eruption occurred in the Motoshirane Pyroclastic Cone Group, where limited disaster prevention measures had been taken and was adjacent to a popular ski resort, one person was killed; infrastructure, including gondola lifts, was extensively damaged by ash and ballistic blocks. We conducted field investigations immediately after the eruption as a joint research team. In this chapter, we describe the geologic features of the craters and eruption products, total discharged mass, flow generated during the eruption, and damage to structures caused by the eruption. Generally, deposits from small phreatic eruptions are quickly lost; thus, basic data on eruption products and damage are lacking. However, detailed data were obtained in this survey because the ashfall was deposited between snow layers, ballistic blocks were easily identifiable, and we utilized gondola lift surveys and live camera movies. This investigation provides basic data for elucidating the mechanisms of small-scale eruptions and formulating disaster prevention measures.