In the early 1930s, a flat rotation curve V(r) that does not follow Kepler's law was discovered in spiral galaxies, including the Milky Way, and became a major problem that shook up cosmology. This is also a gravitational problem that humans will encounter when they leave the solar system in future space development. Initially, this phenomenon could not be properly explained by Newtonian mechanics, and since then, the hypothesis that an unidentified substance that increases gravity, "dark matter," exists has been proposed as the mainstream model. This model has been supported by indirect research on cosmic microwave background radiation and gravitational lensing effects, but conclusive evidence of the existence of dark matter has yet to be found. 　In order to solve this problem, this study focuses on the characteristic shape of galaxies and proposes a new "spiral gravity model (SGM)" based on Newtonian mechanics. In this model, I predict the existence of an interstellar force T(r) generated by the spiral shape as a new centripetal force, and for the first time succeeded in deriving an analytical solution V(r) that incorporates shape data. Furthermore, this analytical solution V(r) was confirmed to be in perfect agreement with the observational data for the entire region from the edge of the bulge to the solar system within ±5%, leading to the verification of the flattening properties. 　From these, I discovered that the centripetal force of the disk is a new force, the "spiral force," which includes the conventional gravity, by adding the force of the spiral motion. This spiral force is proportional to the -1/2 power of the distance r from the galactic center, and decays extremely slowly compared to the -2 power of the distance r in the case of gravity, which is the cause of the flattening. I believe that this force is generated by the macroscopic spiral structure of the star group and becomes apparent as the scale expands to the galaxy. This is fundamentally different from Kepler's law, and I believe that it shows a new principle of gravity that is not in the conventional concept. Furthermore, to confirm the validity and development of the new model, I analyzed various characteristics of galaxies based on the derived analytical solution, and obtained many important findings. 　I hope that SGM will be used as an effective analytical tool in the future to understand spiral galaxies more deeply and to prepare for space development in the near future. This book is available in both Japanese and English.