The fatality rate of the coronavirus disease (COVID-19) at the beginning of the pandemic was as high as 8.5%, and it was considered to be an intractable infectious disease. Reports regarding early experiences are essential for improving nurses' quality of care, patient safety, and working conditions during future pandemics. Therefore, this study aimed to describe the experiences of nurses who were in charge of critically ill COVID-19 patients during the early stages of the pandemic in Japan. This was a qualitative study. Participants were nurses who were in charge of critically ill COVID-19 patients in an emerging contagious disease ward between February and April 2020. Interviews were conducted in groups of 2–3 persons based on an interview guide using an online conferencing application to prevent infection. Consent to participation was obtained from 19 nurses. Five categories of experiences were generated from the analysis: "Fear of risk to my own life and to those of others around me", "The shock of finding myself amid an infectious disease pandemic", "Anxiety about unknown challenges", "Driven by a sense of purpose", and "Growth as nurses". Working under harsh conditions where nurses' safety is threatened may affect the quality of care and nurses' mental health. Therefore, nurses should receive both short-term and long-term support.