Helium is present in very small amounts in volcanic gases, and its isotopic ratio (3He/4He ratio) sensitively reflects the state of magma underground. The 3He/40Ar* ratio, where 40Ar* denotes the magmatic 40Ar, also provides information on the degassing state of the magma. In this chapter, the spatial and temporal variations in the 3He/4He and 3He/40Ar* ratios of volcanic and hydrothermal fluids in and around the Kusatsu-Shirane volcano are discussed to clarify how magmatic fluids are incorporated into the shallow hydrothermal system and how their contributions would reflect variations in magmatic activity. The spatial variation in the 3He/4He ratio of the fumaroles and hot/cold spring gases shows systematic 3He/4He decreases with increasing distance from the Yugama crater, where most of the recent phreatic eruptions occurred. The spatial 3He/4He trend can be explained by a hydrodynamic dispersion model, in which crustal helium with a low 3He/4He ratio progressively leaches into an originally 3He-enriched magmatic fluid as it migrates through the local crust. However, the 3He/4He distribution is also controlled by the local permeable structure at shallow depths, resulting in a difference in the residence time of migrating fluids underground. The temporal variation in 3He/40Ar* ratios observed in some active fumaroles between 2014 and 2021 may reflect changes in magma vesicularity, indicating that the 3He/40Ar* ratio of fumaroles is a useful parameter for monitoring the current state of degassing magma, which is essential for understanding the deep processes of volcanic unrest and may contribute to the identification of precursors of future eruptions.