Yugama crater lake is a famous tourist attraction on Kusatsu-Shirane volcano, Japan, known for its breathtaking scenery. However, phreatic repeated eruptions have occurred, resulting in fatalities. The supply of magmatic fluids to the lake is suggested by its extremely low pH; therefore, the lake has been monitored, including its area, water level, temperature, and concentrations of various ions. These parameters change in response to meteorological conditions, meaning that the results of lake monitoring cannot be used directly to assess volcanic activity. Using a numerical model that considers the meteorological conditions, we analyzed the lake observations to evaluate the fluid discharged from the lake bottom. Our numerical results suggest that the lake is maintained by fluid discharged from the lake bottom, not by rainfall. Furthermore, we show that the heat flux and Cl concentration of the fluid from the lake bottom increased from 13 MW and 2700 mg/kg before the unrest in 2014‒2016 to 20 MW and 5300 mg/kg after, suggesting that Yugama crater lake is a sensitive window into the magmatic–hydrothermal system. We propose a simple method for evaluating changes in the concentration of the fluid discharged from the lake bottom using time-series of ion concentrations in the lake water. Lake observation is, in most cases, low-cost and easy; therefore, lake observations are one of the most effective observations for the surveillance of Kusatsu-Shirane volcano.