Measurements of crustal deformation in volcanic regions play an important role in volcano monitoring. With the recent development of synthetic aperture radar (SAR) technology, posteruptive deflation has been observed after phreatic eruptions in various volcanoes (e.g., Hamling et al., 2016;Himematsu et al., 2020;Narita & Murakami, 2018). Volcanic deflation, which occurs at different temporal and spatial scales, is explained by various factors, such as decreases in pore pressure resulting from fluid migration (e.g., Todesco et al., 2014; and thermoelastic responses with cooling (e.g., Furuya, 2005;. Constraining the source of posteruptive deflation is important when evaluating the structure and physical properties of hydrothermal systems beneath volcanoes and assessing the risk of future phreatic eruptions and signals during volcanic unrest. However, the relationship between the deflation source and the structure of the hydrothermal system based on preexisting subsurface surveys has not been sufficiently discussed in previous studies. Recent magnetotelluric surveys have revealed the structure of the hydrothermal system beneath Hakone volcano, the focal point of this study, providing an appropriate context within which to discuss this topic.Hakone is a caldera volcano located approximately 100 km west of Tokyo, the capital of Japan (Figure 1). This volcano has been active for more than 400 kyr, and effusive eruptions of andesitic magma in the past 40 kyr have formed its central cones (e.g., Mts. Kamiyama and Komagatake in Figure 1; Geological Society of Japan, 2007). Since its latest magmatic eruption (3 ka), several phreatic eruptions have occurred near Owakudani, the largest fumarole area of the volcano, which was formed on the foot of the latest edifice (M. Kobayashi, 2008;M. Kobayashi et al., 2006;Tsuchiya et al., 2017). Since the beginning of the 21st century, volcanic unrest has occurred every few years. The unrest that began in April 2015 was the largest in terms of seismicity in the history of modern observation since 1960. The 2015 unrest culminated in a small Abstract From June 29 to July 1, 2015, a phreatic eruption occurred in Owakudani, the largest fumarole area in Hakone volcano, Japan. In this study, an interferometric synthetic aperture radar (InSAR) time series analysis of the Advanced Land Observing Satellite-2 (ALOS-2)/Phased Array type L-band Synthetic Aperture Radar-2 (PALSAR-2) data was performed to measure deformation after the eruption. The results show that the central cones of the volcano have subsided since the eruption and its deflation source is located beneath the previously estimated bell-shaped conductor, which is considered as a sealing layer confining a pressurized hydrothermal reservoir. Therefore, the InSAR results demonstrate the deflation of the hydrothermal system beneath the volcano. One possible cause of this deflation is compaction due to a decrease in pore pressure caused by rupture and fluid migration during and after the eruption.