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This study investigates the process of melt re-injection into a shallow, large magma reservoir after a giant caldera eruption, based on observational evidence. We chose the Kikai Caldera Volcano as our target because it experienced a giant caldera eruption, the Kikai-Akahoya (K-Ah) eruption, 7,300 years ago. High-resolution seismic reflection surveys and analyses of submarine deposits revealed that the uppermost seismic unit is a pyroclastic deposit produced by the K-Ah eruption, with its estimated volume of >71 km³ (Shimizu et al., 2024). The total bulk volume of the K-Ah eruption was then estimated to be 133–183 km³ in DRE (Dense Rock Equivalent), suggesting that it was probably the largest Holocene eruption. To investigate the current state of the magma reservoir beneath the Kikai Caldera Volcano, we conducted a seismic refraction survey using 39 ocean bottom seismometers and an airgun array along a 175 km survey line across the volcano. The results of the 2D P-wave velocity structure revealed a low-velocity anomaly with a reduction rate of over 15% (maximum 22%) directly beneath the volcano, indicating the existence of a large magma reservoir at a shallow depth of 2.5–6 km. This reservoir is approximately trapezoidal in shape, with a width at least equal to that of the inner caldera. The low-velocity anomaly enabled us to estimate the melt fraction to be 3–6%, but it could be limited to 10% at most. Integrating these results with petrological evidence allows us to propose a model of melt re-injection to form a magma reservoir in the same location as the shallow magma reservoir during the giant caldera eruption. The estimated magma depth during the K–Ah eruption period was 3–7 km (Saito et al., 2001, 2003), and the estimated magma depth for the post-caldera central lava dome formed after 3,900 years ago was 2–4 km (Hamada et al., 2023). These depths overlap with the 2.5–6 km depth of the present magma reservoir identified in this study. Furthermore, the central lava dome exceeds 32 km³ in volume and has a different rock composition from that of the K–Ah eruption magma (Tatsumi et al., 2018), suggesting that different melts were reinjected into the magma reservoir at the same location. The model suggests the following scenario: (1) the K–Ah eruption occurred 7,300 years ago, ejecting approximately 160 km³ of material; (2) caldera formation occurred immediately afterwards; (3) after 3,900 years ago, new melt was injected to form a shallow magma reservoir at the same location, with at least 32 km³ of lava reaching the surface to form the central dome; and (4) this is reflected in the current low-velocity anomaly, characterised by increased melting rates in the magma reservoir. This model may demonstrate a common feature of volcanoes that have experienced a giant caldera eruption, and the temporal variation in low-velocity anomalies in the shallow crust is a crucial indicator for eruption prediction.