Geomagnetic field measurements at active volcanoes have been performed to investigate the thermal state within the shallow volcanic edifice. This aims to measure changes in the magnetization structure inside the volcanic edifice caused by heat supply as geomagnetic changes. Measurements are made by repeated/continuous observations on the ground and by aerial surveys using flying objects, each of which has its advantages and disadvantages. At the Kusatsu-Shirane Volcano, repeated and continuous observations around the Yugama crater lake have been conducted by the Kakioka Magnetic Observatory of the Japan Meteorological Agency since the 1970s. The geomagnetic variations up to 2012 obtained from these observations and their causes were reviewed by Takahashi and Fujii (J Volcanol Geotherm Res 285:180–194, 2014). In this chapter, we first introduce the aeromagnetic surveys conducted to estimate the magnetization structure of Kusatsu-Shirane Volcano and its surroundings and present the magnetization structure inferred from the latest survey using an unmanned helicopter after the 2018 eruption of Mt. Motoshirane. Next, we introduce the continuous observations of the total geomagnetic intensity conducted by the Tokyo Institute of Technology since 2010. Since observations began, increased volcanic activity has been observed in 2014 and 2018, including earthquake swarms and ground inflation around the Yugama crater. Distinct changes in the geomagnetic field probably attributed to the thermal demagnetization of rocks within the volcanic edifice have been identified twice in association with these unrest events. The features of these variations were summarized, and the locations of the magnetic sources were estimated using simple model calculations. Combined with the estimated location and other geophysical information, we infer that these variations are related to the ascent and accumulation of hydrothermal fluids.