Although I decided not to continue watching Gifuu Doudou Anime, but I find this instrument is very interesting. Maeda Keiji playing this instrument when Naoe coming to meet him for the first time.
It originally came from China. A Biwa is a lute with a short neck and is held vertically while you play it. The strings on a Biwa range from three to five. Instruments that are played with the Biwa are known as Biwagaku. This particular instrument was used for traditional story telling. As good as it sounds when played mixed with other instruments, it is also used for solos. More info on Biwa – can be read at wikipedia or ottomen.