The Repository stores many precious artifacts, one of which is a bag for a string instrument called the biwa. Judging from the bag’s form, it is thought to have been used to store a four-string biwa. There are currently five four-string biwa stored at the Shōsō-in, but it is not known whether this bag was used for one of these biwa or some other biwa. The sewn threads of the bag in its current state are cut, and the individual parts are no longer connected. The fabric used for the front cloth of the bag is nishiki, featuring a grand karahana design in light blue (hanadaji daikarahanamon nishiki), while the back cloth consists of a white hiraginu plain-weave silk lining. The core of the bag is made of white felt. The light blue grand karahana design nishiki used as the fabric for the front cloth exhibits a weft-patterned compound weave (nuki nishiki) sporting nine colors woven into an intricate design. The standard width of nishiki textiles from the Nara period stored at the Shōsō-in is 56.4 cm. In contrast, the nishiki textile used for this particular biwa bag is believed to originally have been 113.6 cm in width, making it twice as wide as most other textiles. This is a fine example of the high level of craftsmanship evident in the woven nishiki of the period.

In addition, tests conducted in recent years on the white felt used for the core of the bag have shown it to be made of wool. However, during the Nara period there were no sheep in Japan from which wool could be obtained. Based on the width of the nishiki and the fact that wool was used for the felt, it is believed that this biwa bag was brought over from China.