“The general consensus among scholars appears to be that the earliest iron artefacts found in Japan are from the early Yayoi period; these earliest artefacts are imports, but by the late Yayoi period iron weapons and tools are being made locally (see e.g. Kubota 1986; Yoshimura & Barnes n.d.). The same question is relevant here as in Korea: was the raw material for these locally-produced iron artefacts produced locally or imported?
A large number of wrought-iron plates, very similar to those mass-produced by the Han iron industry (see above), have been found in Japan. These are listed by Li Jinghua (1992: 109), who proposes that they are imports from China. I agree with him that this is the most likely explanation for these artefacts.
Hashiguchi Tatsuya (1992: 99-100) reproduces diagrams of two iron-production sites in Kyushu, but gives no explanation. One of these, his figure 1, excavated in Fukuoka, is fascinating, for it appears to show an odd type of bloomery which might be an early ancestor of the traditional Japanese tatara furnace (on which see e.g. Rostoker et al. 1989). It is dated to a time between the late Kofun and the late Nara period. If this is indeed a bloomery it is the earliest I know of from anywhere in East Asia. It shows that bloomeries were in use in early times in the Korea-Japan area, and my guess is that the bloomery iron-production technology was learned from Siberia rather than from China. Its peculiar construction may have been developed in Korea or Japan in response to the technical problems caused by the use of ironsand ore; such problems have been observed in eighteenth-century American bloomeries (Horne 1773) and in twentieth-century Chinese traditional “dwarf” blast furnaces (Wagner 1985: 17, 38, 55, 57).
Hashiguchi (1992: 101) also illustrates several early iron axeheads 
from Kyushu. Some of these closely resemble early Chinese cast-iron implement-caps, while others are obviously of wrought iron. The former he takes to be imports, the latter local products, and I would agree. There is however a peculiar problem here: in each of the “cast iron” axeheads, at the socket end, there is a gap which Li Jinghua (1992) takes to be a sign that these artefacts actually are of wrought iron, and he therefore suggests that they are local wrought-iron imitations of imported Chinese cast-iron axeheads. In Hashiguchi’s sketches these artefacts look very much like cast iron, and it would have required great skill on the part of the smith to imitate the cast implements so closely. The gaps in these artefacts may be cracks which occurred in the casting process: somewhat similar cracks are seen occasionally in ancient Chinese cast-iron artefacts (e.g. Mancheng, 1980: 280-281, pl. 197.1). In modern times, when white cast iron is cast, in order to avoid such cracks, the core of the mould is normally made of a material which is crushable and does not resist the shrinkage of the casting during solidification. In ancient China this type of artefact was often cast in cast-iron moulds with cast-iron cores, and cracking during solidification can be expected to have been a common phenomenon. Examination of the artefacts from Kyushu should solve this problem very quickly.”
Source: Early iron in China, Korea and Japan Roundtable discussion notes by Donald B. Wagner author of The earliest use of iron in China
Heritage of Japan 