This is a Sanbaso mask, or Kokushikijō. The Sanbaso is a dance performed on the noh theater stage, after the Okina dance, which uses the Okina or Hakushikijō mask.
The okina dance seems to have been established as a ritual in the late Heian period (794–1192), and celebrates peace and good harvests. The masks of Okina and Sanbaso are sacred, and even today the dancers will abstain from eating made or drinking alcohol for a certain period of time before wearing these masks. Okina is a white and noble mask, while Sanbaso is a black, sunburned mask, with sparse teeth and a friendly face. The head of the noh troupe is supposed to play the Okina, and the kyōgen actors are supposed to play the Sanbaso. The jaw of the mask is separated into an upper and lower part, tied with a string to make the jaw move. This is similar to the Okina mask. Bits of hair are implanted for the eyebrows, mustache, and beards of these masks.
This mask has a more elegant face compared to other Sanbaso masks, and has soft sculpted wrinkles. These features suggest that this work was made in the Nanbokuchō Period (1336–1392). Over time, facial expressions became more humorous and wrinkles became stronger and sharper. The mask is made of broadleaf trees, and in an X-ray CT image, you can see that the wood grain of the lower and upper jaws was originally connected, suggesting that it may have been cut off with a saw. On the back, "made by Nikkō / Nōsei (Ka'ō)" is written in gold. Nikkō was a legendary mask carver and purportedly skilled at making Okina masks. The Nōjō was the head of the family of the Kita-ryū sect from the late Edo period (1603–1868) to the start of the Meiji era (1868–1912). This mask used to be in the collection of the Umewaka family, Kanze School actors who performed leading roles (shite) on the noh stage.