The Space Transportation System Committee of the Institute of Space and Astronautical Science (ISAS) of the Japan Aerospace Exploration Agency (JAXA) has been continuously drafting medium- to long-term strategies in the research field of space transportation system under the Inter-University Research Institute System of ISAS since FY2018. This committee is also considering the role of ISAS in cooperation with the organization-wide activities of JAXA to formulate strategies in the space transportation system field. Among its previous achievements, the committee assembled a strategic target and scenario for the space transportation system research field at the end of the last fiscal year and is continuously revising it. Based on a formulated mission scenario, the committee identified three priority areas related to system technologies that must be tackled. They are a “reusable orbit transportation system” that aims for a highly frequent mass transportation from Earth to low Earth orbit, “deep space interorbital transportation system” that aims for a marked improvement in space science and exploration missions in terms of frequency and flexibility, and “small flying test bed system” for flight demonstrations, which is indispensable in the research and development of space transportation systems. In this paper, the authors summarize the medium- to long term strategies and their concrete implementation measures over the next two decades.