Minor seismicity may occur at volcanoes with hydrothermal system before a steam eruption. To forecast any steam eruption, it is indispensable to detect and understand the nature of this shallow seismicity. As the fumarolic gas resides in the hydrothermal system, it may provide insights for elucidating the nature of any seismicity and thus forecast steam eruptions. At Kusatsu-Shirane volcano Japan, intense seismic activity took place in 2014 and 2018. To investigate the relationship between the seismicity and gas chemistry, five fumarolic gas discharges have been repeatedly analyzed. Since July 2014 to November 2017 a monotonic decrease in CO2/H2O, He/H2O and N-2/H2O ratios was recorded in the fumarolic gasses located north of the summit of volcano, suggesting the decline of the magmatic component. On the contrary the CH4/H2O ratio significantly increased during the seismically quiet period, indicating that reduced conditions developed in the hydrothermal system, favoring the formation of CH4. The high N-2/He ratio in the quiet period indicates the addition of N-2, likely deriving from the crustal rocks hosting hydrothermal reservoir. The N-2/He ratio in 2018 was significantly lower than those recorded in 2014, indicating the evolution of magma with the progress of degassing. The delta D(H2O) and delta O-18(H2O) values and the CO2/H2O ratios of fumarolic gas discharges were modeled with the following processes: generation of vapor phase after the mixing between magmatic gas and a cold groundwater with meteoric origin, addition of vapor phase with meteoric origin, and partial condensation of water vapor near surface. Only a single magmatic gas is necessary for the above modeling. These data suggest that at Kusatsu-Shirane volcano the activation of seismicity was synchronized with the increase of the magmatic component in the fumarolic gas. It is postulated that the injection of magmatic gas increased the fluid pressure in the reservoir, which triggered seismicity. The injection would have been triggered by a break of the sealing zone surrounding the degassing magma. The injection of magmatic gas can be detected by monitoring the composition of the fumarolic gas, thus giving the possibility to forecast any future seismicity.