1) The aim of this study is to investigate the relations between the “public investment” and the regional economy which is remarkably changing, being influenced by the “high-pitched growth” of our national economy since the middle of 1950. Aichi Irrigation System Plan is one of the good examples of these public investments on the development of regional economies undertaken by our government under the law for the “All-round National Land Planning”. Aichi Irrigation System covers about 30, 000 ha. of farm land near Chûkyô Industrial District. The industrial production of this District has come to occupy a larger part of the whole national production than ever, and with the construction of combinats at the southern part of Nagoya city, the District holds more heavy-and-chemical industry than ever. As the result of this process, the industry will have an enormous demand for water and will expect Aichi Irrigation System to supply it. But the main function of the System is to contribute to the agricultural production of this area, as far as written on the paper plan. 2) In the agricultural district covered by the Irrigation System, most agricultural families own only 0.5_??_1.0 ha. of farm each. The farmers endeavored to increase their products for several years after World War II, when Japan suffered from food shortage and the prices of food were very high. And they supported the plan of the Irrigation System. But since Korean Incident, Japanese industry has been quickly reconstructed, and labor market has begun to spread. Since 1955-56, the cutflow of laber forces from agriculture has remarkably increased. In 1950, most farmers' families followed only agriculture in almost all the communities covered by Aichi Irrigation System. But in 1960, as the result of the outflow of labor forces especially from the farmers' families of smaller-scale management, the ratio of the families that take their farming for a side work became to correspond to the average scales of farming managements in each community. On the other hand, food trade was decontrolled (except rice) in 1952, and the demand for food was increased and became various. Then agricultural managements were considerably commercialized. In the Irrigaticn District, too, many farmers, above all, farmers of larger-scale management, have come to raise fruits and vegetables for the market of Nagoya and other cities. The growth of Chûkyô industry has not only promoted the outflow of labor forces from agriculture but has urbanized the Irrigation District itself. And the communities such as Ôdaka, Arimatsu, Idaka and Narumi were excluded from the Irrigation System. Such exclusions may spread over other communities. 3) The farmers who have their farm in the Irrigation District must pay their allotment (about 400-800 thousand yen per ha.) to the government for the cost of the construction of the System. Can they make the payment as an effective investment for their farming business ? Anb can they keep labor forces necessary for the irrigating work ? By means of the Irrigation System the paddy field can be improved so as to make it easy for the farmers to use cultivating machines and reduce the labor hours. For the families that have less than 0.7 ha., however, it is almost impossible to buy the machines, and as the increase of crops per ha. cannot be expected so much by this improvement, the Irrigation System brings them only higher producing cost of rice. Some of them must give up their farms when they are compelled to pay the allotment. And such families are the majority in this District. Most families who have over 0.7 ha. will utilize the reduced labor for the non-agricultural work. Some families of larger scale management, probably those who have more than 1.5 ha. may utilize the reduced labor for raising fruits, vegetables and other crops.