In the past about forty years, with “understanding the hot Universe from its origin to the present” as a goal in my mind, I have conducted researches in three different “phases” of space astronomy and astrophysics: (1) observational studies of the universe at X-ray wavelengths using instruments available at the time, (2) research and development of new instruments to answer questions raised from (1), (3) development of space missions carrying new instruments from concept study to realization. I also contributed improvement of concept studies of space science missions at Japan Aerospace Exploration Agency (JAXA).