This paper outlines Matsui's career, and clarifies the characteristics of his architectural activities in I) Tokyo, II) Milano, and III) Kagoshima period. Matsui's career I) After graduating from the Tokyo University of the Arts in 1956, he joined the Isamu Kenmochi Design Institute and was promoted to director and chief designer in 1963. He collaborated with Mikami and Sugiura, who were his classmates at the university, and this relationship continued until his later years. He was also engaged in educational activities as a part-time lecturer at Musashino Institute of Technology from 1965. In particular, being worked at the Kenmochi Design Institute was a turning point for Matsui, as he met Gregotti. II) Matsui participated in the Gregotti office, which had begun activities in Milano since 1968, and came to play an important role as a partner architect when Gregotti Associati was established in 1974. He also was involved in education with Gregotti, who was a professor at the university. Matsui sought to obtain an Italian architect qualification (Architetto), but although he passed the qualification test, he was unable to register due to nationality issues, which prompted him to return to Japan. It is speculated that his experience in educational activities in Italy and the fact that the Department of Architecture of Kagoshima University at that time planned to invite a professor architect, led to start for his career as a professor at Kagoshima University after returning to Japan. III) In 1983, he joined the Faculty of Engineering, Kagoshima University as a Professor of the Department of Architecture, retiring in 1997. He was also appointed to the Kagoshima Economic University in 1999 and retired in 2003. In his later years, he was also active as an artist. Matsui's architectural activities I) Matsui was in charge of the interior and furniture design, especially he designed furniture and fixtures with same height using modules, as well as color schemes. In this period, Matsui cultivated Kenmochi's design ideas, such as the importance of understanding the sense of scale and conveying details using the actual size drawing. In addition, through the collaboration with Kunio Maekawa and Sachio Otani, various architectural ideas and design methods were cultivated in Matsui, and it can be inferred that he had an integrated view of interior design and architecture. II) Under Gregotti, Matsui was involved in large-scale projects such as city planning in addition to architecture, and the scope of design for him had expanded to the urban scale. Simultaneously he had a great influence on the projects and management of the office with an accurate understanding of Italian architectural culture. He also worked independently recognizing the importance of his own individual activities. The works during this period were characterized by linear composition, grid composition of square modules, using of squares in plan, elevation, and openings, aligning the top edge of building against the different ground level. III) Matsui left behind three implementation works that are reminiscent of the works of the Milano period, but also show his unique expression using the Trompe l'oeil and color painting. He was involved in research activities related to Italy and also served as a member of urban planning and other committees. In addition, Matsui contributed for the entire period, serving as a bridge between Japan and Italy to introduce each other's architectural culture.