<論説>法定鋳銭額からみた平安前期の周防鋳銭司

Abstract

本稿は、天長二年(八二五)に周防国吉敷郡に成立した鋳銭司(周防鋳銭司)について文献史料を中心に検討する。鋳銭司が一年間に鋳造すると定められた銭貨の額(法定鋳銭額)は、鋳銭司の活動規模を知る上で重要なバロメーターで、先行研究でも変遷が議論されてきた。本稿は史料を再検討し、従来の説とは異なり、原料に旧銭を加えることで、九世紀前半には一貫して一万一〇〇〇貫文鋳造体制をとり、九世紀末から一〇世紀にも三五〇〇貫文鋳造体制を維持したことを指摘した。また鋳銭の原料は鋳銭額に応じて算定されるが、その計算式は九世紀初頭から一〇世紀まで変動しないことを明らかにした。この間、原料に旧銭が加わったことで、帳簿と実態の乖離が生じたと思われるが、銭貨の小型化は、鋳銭司による帳簿と現実のすり合わせと見ることもできることを論じた。

The former provinces of Suō and Nagato, regions of present-day Yamaguchi Prefecture, were major mineral mining production centers in ancient Japan. Nagato Province with its a vast karst topography, produced abundant mineral resources such as copper and lead, and supported national projects such as the creation of the Great Buddha and the minting of coins. This paper chiefly relies on written historical sources to examine the mint known as the Jusen-shi 鋳銭司, or Suō Jusen-shi, which was established in 825 in the Yoshiki 吉敷district (present-day Yamaguchi City) in Suō, following its relocation from Nagato. During the Heian period, the Suō Jusen-shi was the only central government institution that continuously minted coins, but this mint was in Suō Province far from the capital and the Kinai region. Ancient Japanese coins were made of copper, and the mint was established there due to its proximity to the source of the raw material. The Suō mint also serves as an important example when considering the ideal form of government offices and production facilities located in outlying areas. In considering the trends in the organization of the mint in the Heian period, the scale of production, which is closely related to the size of the government office and the number of staff working there, is an important barometer in determining how many coins were minted in a year, and the changes in these numbers has been debated in previous studies. When the historical sources related to these numbers were reexamined, it became clear that the change in the number of coins minted differed from previous estimates. Previous studies had argued that due to a shortage of copper, an edict issued in 841 reduced the annual production from 11, 000, 000 to 3, 500, 000 coins. However, re-reading this edict and related historical documents makes clear that the number of coins produced did not change. In addition to newly mined copper, older coins were also retrieved and used as raw material so that 11, 000, 000 coins were still made every year until the middle of the 9th century. It was not until the end of the 9th century that the number was reduced to 3, 500, 000. I also found that the Suō mint had a formula to calculate the amount of copper and lead necessary to make copper coins. The formula was much more precise than previous studies had assumed. By using this formula, the officials of the Suō mint were able to calculate the necessary amount of raw materials very accurately. Moreover, it became clear that this formula continued to be used unchanged from the 9th century to the 10th century. However, when we observe Heian period coins today, we see that the size of the coins became smaller as time passed. In other words, the amount of copper and lead used in each coin decreased. The reason for the discrepancy between the amount of copper and lead needed according to the calculations and the actual number of coins produced is thought to be that in addition to newly mined copper, copper from old coins was also used. It appears that it was necessary to reduce the size of the coins in order to maintain their number because it was not possible to take as many coins as expected from the older coins.