The Aizu Miyoshi line took root in Iwashiro when the Katō house was transferred from Iyo to Aizu in Ōshū in 'ei 4 (1627). The trace the family back to Nagakuni, a smith who had resided at Matsuyama in Iyo Province and entered the service of Katō Yoshiaki; when his patron moved north, Nagakuni followed in the retinue, and his son, the first-generation Masanaga, relocated with him. Masanaga's eldest son, born in Aizu in 'ei 10 (1633), became the smith the records overwhelmingly cover: Miyoshi Nagamichi, also called Miyoshi Tōshirō. He first signed Michinaga (the entries also note early signatures read Dōchō and Dōnaga), studied under his grandfather Nagakuni, and in Manji 1 to 2 (1658 to 1659) travelled to Kyoto, where he received the court title Mutsu Daijō and changed his name to Nagamichi. He died in Jōkyō 2 (1685) at the age of fifty-three. The repeatedly name him as the de facto founder of Aizu , working under the patronage tied to the Aizu Matsudaira house, several of whose presentation blades entered the Imperial Collection.
Across the blades the hand is consistent. The forging is , often mixed with , , and that runs toward , with adhering thickly and fine entering; the tends to a shallow , called particularly shallow even among work in the notes. The temper opens with a short, -based , then broadens into mixed with ; in places two run together as a gourd-shaped hyōtan-ba. Long enter, the is deep, and adheres thickly, sometimes coarsely as or unevenly as mura-, with and worked into the and a bright . The runs in or to with at the tip, the type the records call a "Kotetsu ." The tie this manner directly to the "" phase of Nagasone Kotetsu in , the recurring point of comparison; one entry also records a tradition that Nagamichi was a disciple of Tsuda Sukehiro, though the writers hold his work closer to the Nagasone group.
For , the markers are the shallow , the hyōtan-ba pairing of , the and over deep , and the Kotetsu-type , balanced against the smith's tendency toward coarser . The dated blades range across Enpō 3 through Jōkyō 2 (1675 to 1685), the last corresponding to the year of his death and valued as documentary material on his final years. Cutting-test inscriptions by Yamano Kanjūrō Hisahide appear in gold inlay on several . The provenance recorded is notable: blades presented by the Aizu Matsudaira family entered the , and one is held by tradition to have been bestowed on Kondō Isami of the Shinsengumi by the Aizu lord Matsudaira Katamori in recognition of the Ikedaya Incident, later passing to the Meiji general and connoisseur Tani Tateki and thence into the Tosa Yamauchi family. The register places Nagamichi as the smith who set the Aizu idiom, his line carrying the Miyoshi name in Iwashiro.