One striking feature of the history of Sung commerce is known as the coming to the fore of the ya-jen 牙人. The ya-jen were originally free agents who mediated between the dealers and earned a commission for doing so, but during the Sung dynasty they also functioned as middlemen who traded in commercial goods themselves. This article will try to look into the changes of the market organization during the T'ang and the Sung from the differentiation and expansion of the functions of the ya-jen. It will also examine the changes in state concern towards commerce, starting from the changes in policies of the Sung Court towards the ya-jen. Regarding first the relations of the ya-jen with the Sung Court, the ya-jen who were employed in finances increased rapidly after the "state trade system policy" (shih-yi-fa 市易法). Their job was the investigation of the quality of the goods purchased by the government and the estimation of their value, and it was based on their mediating function. These ya-jen were probably called "governmental ya-jen" (kuan-ya 官牙), but during the Southern Sung also the normal, that is "private ya-jen" (ssu-ya-jen 私牙人) began to be involved in the economic policies of the Sung Court. Especially the local officials used the ya-jen, who were familiar with the market, in implementing the trade policy in the area under their jurisdiction, and the financial policies imposed by the central government. And in this case the function of the ya-jen used also included, as we can see in the rice trade, the middlemen-function. If we look back at the process during which the ya-jen evolved from commission agents to middlemen, we are able to infer from the sources that the ya-jen middlemen had as their object the daily consumption goods which were sent from the villages to the cities. That is, the changes in the economic relations of the villages with the cities in the T'ang and Sung periods produced the ya-jen middlemen. The former trade organization which was maintained by means of market regulations broke down, and instead the ya-jen began to take care of the trade between the villages and cities. The state at first considered the ya-jen middlemen as undesirable but, in the end, acknowledged their importance and used them for its trade policies. Next I investigate how the Sung Court changed its methods of commerce control in answer to the trade development at that time, which we can discern through the evolution of the middlemen-function of the ya-jen. After the dissolution of the market regulations which restricted the locations of trade, the shih-yi-fa was enacted. The shih-yi-fa didn't differ from the market regulations in as far as it directly controlled commerce, but it did differ in that it had as its target for control the trade process as such. But the shih-yi-fa failed. The Sung city markets didn't allow for the direct intervention of the state. Afterwards the ya-jen were the core of the commerce control adopted by the Sung Court. The ya-jen organizations of the Southern Sung were in the position of controlling the prices as a vital link in trade ; but the Sung Court had the sole power of approval for the foundation of such organizations. So, the Sung Court abandoned their direct grasp on trade at which the shih-yi-fa aimed, and carried out an indirect command by controlling the ya-jen.