Bitasen was a coin of very poor quality among those circulated during late Medieval Japan. They were also called akusen.
In Japan money circulation had become active around the late Kamakura period, and Chinese coins minted and used in China were predominantly circulated. These Chinese coins were brought into Japan through trade with China (during the Baisong, Yuan Dynasty, for example) and such coins also came to be privately minted in Japan. These coins were called as Shichusen (private coinage). The quality of Shichusen was often very poor, for example part of the detail was often missing, it had no hole, and the letters on it were impossible to read. Thus, they tended to be disliked in the commodity economy. Such poor quality coins were called bitasen and considered to have less value than common coins.
As the Muromachi period began and Eiraku-tsuho (bronze coins struck in the Ming dynasty) minted in China during the Ming Dynasty for trade with Japan were circulated in Japan, Shichusen minted in Konan (Jiangnan) China and Shichusen minted in Japan were gradually mingled together. Not only Shichusen, but wartime coins of the Chinese Southern Sung Dynasty and Minsen (bronze coins produced during Ming Dynasty) mingled with poor quality coins and were usually of poor quality, therefore they were collectively called akusen. Also, the akusen was sometimes set to lower price compared to 'Seisen' (refined coins) of good quality, or often refused to be accepted. This selection procedure was called Erizeni. Sometimes Erizeni caused bloodshed. However, the turn-over volume of poor quality of Torai-sen (the importation of currency from China) and Shichusen increased. It was criticized that the coins contained mostly akusen, the coins which Yoshihide ASHIKAGA, the 14th seii taishogun (literally, "great general who subdues the barbarians") of the Muromachi bakufu presented to the Imperial court as a reward for the assumption of the Shogun, and coins which Nobunaga ODA presented at the ceremony of genpuku (celebration of one's coming of age) for Imperial Prince Sanehito, the chokun (crown prince) of Emperor Ogimachi. So many akusen circulated that even powerful persons could hardly obtain good quality coins, and Yoshihide was forced to disrespect the Imperial court.
Thus, in the sixteenth century the Muromachi bakufu, Shugo daimyo (Japanese territorial lord as provincial constable), and daimyo (Japanese territorial lord) during the Sengoku Period tried to officially announce a ban on the Erizeni to achieve smoother money circulation. But there a deep-rooted consciousness existed among common people to avoid bitasen. As the Edo bakufu put Kanei Tsuho (coins) of stable quality into circulation and strongly prohibit Torai-sen and Shichusen, akusen disappeared and Erizeni was not carried out any more.