<p>Clarifying the formation process of a high-temperature reservoir in a post-caldera-hosted geothermal system is critical for specifying areas with potential for power generation. This clarification also contributes to evaluating heat source and geological structure controls on heat and fluid flows and temperature distribution in the reservoir. This study focused on a high-temperature, liquid-dominated reservoir and aimed to clarify its formation process through a precise numerical reservoir simulation. The Karurusu geothermal field in northern Japan, with six exploration wells, was selected for a case study of a typical post-caldera system. The simulation emphasized the bottom boundary condition and permeability distribution as strong influences on heat and mass transfer and the formation of a high-temperature reservoir. The small root mean square error (5.4&#xa0;°C) between the calculated and well-logging temperature data confirmed the reliability of the numerical model calibration. The results showed an overpressure condition in the liquid phase of the reservoir, thermal intrusion of 300&#xa0;°C from the heat source, temperature reversal in the outflow, and three required stages to form the high-temperature reservoir: initial, transition, and stabilization. Important geological structures were the development of a caldera wall with low permeability and a dome-shaped caprock, which sealed and trapped the heat within the center of caldera and controlled the magnitude of the upflow plume in the northern region. By contrast, a pathway was developed for the downward recharge fluids in the southern region. The simulation results refine the previous conceptual model of the Karurusu geothermal system, contribute to an effective exploration of high potential zones, specify the essential factors needed to form a high-temperature reservoir, and estimate the time necessary to complete the formation.</p>