It is one of the silk fabrics. This is the traditional Japanese kimono of Hiraori (shantung silk) using yarn-dyed thread of Mawata- thread (Yuuki-Tsumugi) or Tama- thread.
Tsumugi is worn in fashionable and casual way, but not as a formal wear. Nowadays, however, Houmongi and Tsukesage of Tsumugi are made to wear at party.
Tsumugi is woven throughout Japan. Some places are the famous area of production for traditional Japanese kimono called Oshima-tsumugi, Yuki-tsumugi or Shiozawa-tsumugi. There is solid color Tsumugi, but patterns of Kasuri, Shima and Koushi are much more famous. It is technically elaborate work to weave out pattern of Kasuri.