Biwabokuboku is a Japanese specter that is introduced in "Hyakki Tsurezure Bukuro" (One hundred bags in idleness), a collection of specters illustrations by Sekien TORIYAMA, and it is a specter of biwa (Japanese lute).
It is a biwa which has transformed into a biwa-playing minstrel, and a kind of Tsukumo-gami (gods to a variety of things). The body looks like a human being, but its head is biwa, and it carries a walking stick with eyes closed like the blind biwa-playing minstrel.
According to the "Kinpisho" (Records of the history and origin of imperial court ceremonies which set forth the rules and etiquette for carrying out such ceremonies), a practical guide to Yusoku kojitsu (court and samurai rules of ceremony and etiquette) written by Emperor Juntoku, there were once famous biwa called "genjo" and "bokuba." "Genjo" in particular were involved in strange occurrences, such as when a "genjo" dashed out of the Imperial Palace by itself when the place was burnt down and when an ogre, who was attracted to its sound, stole a "genjo" and hung it by a piece of string from Suzakumon (the main, southern gate of the Palace City). According to the explanation in "Hyakki Tsurezure Bukuro", Sekien states that it had been those "genjo" and "bokuba" that have transformed into Biwabokuboku.
Furthermore, since the "Hyakki Yagyo Emaki" (a picture scroll of the demons' night parade drawn in the Muromachi period, which was before Sekien's time) contains a drawing of a biwa specter, there is a theory that Sekien drew 'Biwabokuboku', which is introduced in the "Hyakki Tsurezure Bukuro", based on the "Hyakki Yagyo Emaki."