A number of economically active people in the Third World today earn their livlihood by doing such informal activities as scavenging, street-hawking and so on. It is estimated by the I.L.O. that the average scale of the informal economy in the non-agricultural sector of peripheral developing countries is not less than 50% in terms of employment. Furthermore, it is noticiable that its scale has not been made smaller along the process of industrialization in many of those countries. This phenomenon is indeed in opposition to the underlying assumption attached to the main current of development economics whose basis is on the historical experiences of industrialized countries. This article tentatively clarifies the structural background of formation and conservation of a considerable scale of the informal sector in peripheral economies, within the framework of their industrializing process. Though many researches have been thus far done concerning the actual situation of the informal economy, the structural background of the phenomenon mentioned above is not yet sufficiently made clear, and the essence of the problem of the informal economy is apt to be considered merely as a temporary malfunction of the modern labour market in those countries. On the contrary, this article demonstrates through a critical review of the precedent works, that its conservative nature is attributable to the peripheral mode of capital accumulation which has been in many ways conditioned by the Capitalist World System.