We review the geophysical observations that have been conducted at Kusatsu–Shirane volcano since the 1930s. The possible mechanisms of T-type earthquakes (i.e., monochromatic earthquakes) identified by a seismic network at the volcano are thought to be oscillations and resonances of a crack filled with two-phase water containing solid particles. Borehole-type seismometers and tiltmeters around the Shirane Pyroclastic Cone Groupe (SPCG), which have been operated by the Kusatsu–Shirane Volcano Observatory of the Tokyo Institute of Technology (Reorganized to Institute of Science Tokyo in October 2024) since the 1990s, have successfully observed unrest during 2014–2021, including the phreatic eruption in 2018. During the unrest, a shallow hydrothermal reservoir undergoing gentle inflation was detected beneath the SPCG. During the phreatic eruption in 2018, we found a subvertical crack-like conduit beneath the Motoshirane Pyroclastic Cone Group (MPCG). The geophysical and geochemical observations suggest that a deep hydrothermal reservoir covered by a self-sealing zone underlies the entire Kusatsu–Shirane volcano at ~ 0 m asl. The shallow hydrothermal reservoir beneath the SPCG acts as a buffer against fluid supplied from a deep hydrothermal reservoir, causing unrest such as ground deformation. In contrast, pressurized hydrothermal fluid in the deep hydrothermal reservoir can ascend rapidly to the surface through the crack-like conduit beneath the MPCG, which is one reason why precursors to phreatic eruptions in the MPCG are difficult to detect.