Sukenaga (助長) was a smith of the Bizen Province Osafune group, active during the late Kamakura period around the Kagen (1303--1306) and Showa (1312--1317) eras. Although his lineage has not been clearly established, the NBTHK observes that, judging from his manner of work, he was likely one of the disciples within the circle of Nagamitsu. Sword reference compendia (meikan) record smiths bearing the same characters under the Fukuoka Ichimonji, Yoshioka Ichimonji, and Osafune lineages; however, the existence of a tachi with an ubu-nakago bearing the full inscription "Bizen no Kuni Osafune-ju Sukenaga saku" and dated Showa 1 (1312) conclusively establishes his affiliation with the Osafune school. The same characters, read as Sukecho, also appear among the Ko-Hoki smiths of an earlier generation descending from Yasutsuna, but the Osafune Sukenaga is understood as a distinct identity whose stylistic current resonates with contemporaries such as Nagamoto and Sanenaga.

Sukenaga's characteristic work centers on a suguha-based hamon into which ko-choji, ko-gunome, and ko-notare are mixed, with ko-ashi and yo entering well. The nioiguchi consistently tends toward shimarigokoro — a tightened, restrained quality — executed predominantly in nioi-deki with a slight adherence of ko-nie. His forging displays closely packed ko-itame mixed with mokume, upon which vivid midare-utsuri stands out with particular clarity. The boshi characteristically assumes a shallow notare turning back in ko-maru, at times approaching yakizume in feeling. Nearly all examined works bear bo-hi carved kaki-nagashi on both sides. The NBTHK notes that, while his workmanship presents an exceptionally close resemblance to the Nagamitsu lineage, the calligraphic style of the signature does not reflect the mainstream Osafune smiths, suggesting a peripheral position within the school.

The NBTHK repeatedly characterizes Sukenaga as a smith of high technical ability whose works are both precious and reliable as documentary material. His signed works are recognized as exceedingly rare among extant examples, and the dated Showa 1 inscription is regarded as being of great value for establishing his identity and period. One blade formerly in the possession of the Tokugawa shogunal house carries a sayagaki recording its bestowal by Lord Takechiyo in Bunkyu 2 (1862). Across his oeuvre, the elegant tachi-sugata — slender, with koshizori, funbari, and ko-kissaki — is described as dignified and superb, and his workmanship is consistently assessed as presenting one important stylistic mode of the late-Kamakura Osafune school.