In both 1990 and 2014, the increase of Cl− concentration and the decrease of pH occurred in the lake water of the Yugama crater, located at the summit of the Kusatsu-Shirane volcano. These changes suggest the following sequence of events: a rupture of the sealing zone formed over the solidifying magma chamber, the subsequent intrusion of groundwater into the sealing zone, the extraction of HCl from the hot rock body within the sealing zone, and the transport of HCl to the Yugama crater. We define the volcanic activity cycle as follows: A volcanic activity cycle begins when a sealing zone ruptures and ends just before the next sealing zone ruptures. Therefore, three cycles were identified between 1970 and 2021. Over the first cycle, phreatic eruptions occurred in 1976, 1982, and 1983. During these eruptions, the CO2/H2S ratio of the fumarolic gas adjacent to the Yugama crater increased, suggesting that magmatic gas was injected into the shallow hydrothermal system led to the eruption. The second cycle began in 1990 and continued until 2013 with no phreatic eruptions. The third cycle began in 2014 and continues to the present day (2022), with a small phreatic eruption which occurred ~ 2 km south of the Yugama crater in 2018. The most relevant parameter for phreatic eruptions at the Kusatsu-Shirane volcano is the CO2/H2S ratio of volcanic gases, and a regular monitoring of the chemical composition of the volcanic gases helps predict future phreatic eruptions. The long-period monochromatic earthquakes observed at the Kusatsu-Shirane volcano may be related to an inferred plate-like crack beneath the Yugama crater. We propose a model in which a plate-like crack was formed from the point beneath the crater to its northern foot to allow fumarolic gases to discharge to the ground surface. The inclination of the crack enabled the gravitational separation of the vapor and liquid phases.