Mumon ginsen (無文銀銭) (aka "Plain silver coin") is believed to be the earliest form of Japanese currency. Issued between 667 and 672 AD during the Tenji period, there are theories that mainly lean towards these coins being privately minted.

Throughout their search history stretching back to the Enkyō period, about 120 Mumon ginsen have been excavated from 17 sites: seven in Yamato, six in Omi, and one each in the Settsu, Kawachi, Yamashiro, and Ise regions. ^[a] Each coin is roughly 3 centimeters in diameter, 2 millimeters thick, and weighs about 8 to 10 grams. ^{[1] [b]} These coins differ from later issues as they feature small round holes as opposed to a large central square. ^[1] Only a few of those excavated have inscriptions, and many of the coins have silver pieces attached to their surfaces which are thought to have been added to make the weight uniform. ^[1] All of the coins are thought to have been issued during the reign of Emperor Tenji during the Tenji period based on a written account in the Nihon Shoki. An entry for 683 AD states: "Summer, 4th month, 15th day. The Emperor made a decree; saying:-"Henceforth copper coins must be used, and not silver coins". ^[2] This decree meant that the coins were issued before the Huhonsen. ^[3] Another indicator of dating is the large amount of coins that were excavated from Ōmi Province which suggests a connection to Ōmi Ōtsu Palace. ^[4]

In contrast to Japan, silver coins were not used on the Korean peninsula during the Tenji period. Professor Fumio Tanaka ( Waseda University) mentions that in Silla, Baekje, and Goguryeo, the use of gold and silver was regulated by royal authority. Tanaka went on to say that it's been proven by literature and archaeology that they functioned as a symbol of the royal authority's status order. ^[5] When compared to Japan at the time, Tanaka mentions that before the 7th century gold and silver were supplied by the international community. As each powerful clan (chief) obtained these precious resources, there was a basis to circulate coins in a multilateral manner without being regulated by the royal authority. ^[5]

The true purpose of Mumonginsen has been debated by several historians and scholars since at least the Taishō era. There is now a consensus that Mumonginsen at the very least were recognized as having value as currency.

Japanese historian Ginzo Uchida (1872-1919) suggested that the silver coins dated to the Tenmu period, and were likely not officially minted coins made by the government. Instead he theorized that the coins were made by private individuals in Japan using silver produced in Korea. ^[6] Japanese historian Shinji Nishimura (1879-1943) referred to entries in the 12th year of the Hakuhō period for the silver mining in Tsushima. While he came to the conclusion that the coins may have been used as a type of ornament, the amount of "ornaments" found (100+ pieces) made it difficult to understand. He also suggested that it would be better to think of them as a type of currency. ^[7] Kanichi Kuroda of the Oriental Numismatic Association argued that Mumonginsen were coin-shaped items with a small circular hole. He then stated that there was no reason for the state to cast such a crude coin when minting them for the first time. ^[8] Historian Aoyama Reishi stated in his opinion that even if they weren't currency, Mumonginsen must be recognized as transitional circulating coinage. In his reasoning the approximately 100 excavated coins have little variation in weight, and there are too many of them to have been buried as offerings at Shinpo-ji (眞寳院の). (see: "Excavated examples" section below) ^[9] Japanese archaeologist and professor Keiji Matsumura points out that the remains at the ruins sites (where the coins were found) has generally been either discarded or deliberately buried. Since it's difficult to imagine that something as valuable as silver would have been discarded, he suggests that the undeclared silver coins are biased toward those used as altar implements which wouldn't mean they were numismatic charms. ^[4]

• List of Japanese cash coins by inscription
• Kaiyuan Tongbao - concurrent Chinese coinage.