Japan has the highest proportion of older adults worldwide but has fewer critical care beds than most high-income countries. Although the COVID-19 infection rate in Japan is low compared with Europe and the United States, by the end of 2020, several infected people died in ambulances because they could not find hospitals to accept them. Our study aimed to examine the Japanese healthcare system's capacity to accommodate critically ill COVID-19 patients during the pandemic. We created a model to estimate bed and staff capacity at 3 levels of pandemic response (conventional, contingency, and crisis), as defined by the US National Academy of Medicine, and the function of Japan's healthcare system at each level. We then compared our estimates of the number of COVID-19 patients requiring intensive care at peak times with the national health system capacity using expert panel data. Our findings suggest that Japan's healthcare system currently can accommodate only a limited number of critically ill COVID-19 patients. It could accommodate the surge of pandemic demands by converting nonintensive care unit beds to critical care beds and using nonintensive care unit staff for critical care. However, bed and staff capacity should not be expanded uniformly, so that the limited number of physicians and nurses are allocated efficiently and so staffing does not become the bottleneck of the expansion. Training and deploying physicians and nurses to provide immediate intensive care is essential. The key is to introduce and implement the concept and mechanism of tiered staffing in the Japanese healthcare system. More importantly, most intensive care facilities in Japanese hospitals are small-scaled and thinly distributed in each region. The government needs to introduce an efficient system for smooth dispatching of medical personnel among hospitals regardless of their founding institutions.