Windmill irrigation is a way to irrigate paddy fields and upland fields for the purpose of watering and fertilizing. Wind turns a windmill and the power is transmitted to hand pumps which lift the underground water for irrigation. Irrigation for fertilization is conducted by lifting underground water containing nitrogen. Farmers used windmill irrigation on the southern margins of Lake Suwa, on the outskirts of Sakai City, at Higashiura-machi on the Chita Peninsula, at Cape Irako on the Atsumi Peninsula, in the drainage basin of Sakura River around Tsuchiura City, and on the alluvial lowland around Tate-yama on the Boso Peninsula. By studying these areas and comparing the results, the follow-ing findings were obtained.<br> 1. Windmills used for irrigation consisted of two types. In first type, the windmill could only be turned by the wind coming from one direction. The second type could automatically adjust itself to whichever direction the wind blew. The design and type of materials used differed depending on whether a blacksmith or a carpenter controlled the manufacturing of the windmills.<br> 2. The purposes of windmill irrigation were to fertilize paddy fields, to water paddy fields and to water upland fields. Of the three, the watering of paddy fields was the most important.<br> 3. It was from 1905 to the latter half of the 1930s when windmill irrigation was practiced on the southern margins of Lake Suwa (for fertilizing paddy fields), and it was from the first half of the 1930s to the first half of the 1960s when windmill irrigation was used in the other areas (for watering of paddy fields and upland fields).<br> 4. Farmers used windmill irrigation to fertilize their fields as a substitute for fish fertilizers and soybean cake. After Japan started mass production of ammonium sulphate, how-ver, windmill irrigation for fertilizing was abandoned.<br> 5. Windmill irrigation for watering became technologically available after handpumps with cast cylinders emerged, and economically possible after sufficient capital was accumulated by the development of commercial agriculture, mostly sericulture. The reasons why he windmill irrigation for watering ceased to exist were that 1) small-scale engines and pumps became readily available to farmers due both to a decline in their relative prices mnd to the subsidization of their purchase by central and local government; 2) it became lifficult to secure labor for maintenance and management of the windmills; and 3) newly-ieveloped engines were lighter which permitted them to be moved more easily.<br> 6. All windmill irrigation required sea breeze or lake breeze to move windmills. In mddition, windmill irrigation for fertilization required a steady outflow of natural gas which contained nitrogen, that for the purpose of watering of upland fields needed highly productive agriculture, and that for watering of paddy fields was adopted in only areas where, to cope with water shortage, farmers irrigated fields by lifting underground water using sweep-well buckets and they were able to accumulate capital as a result of the development of commercial agriculture. Therefore, the areas adopting windmill irrigation were restricted.<br> In conclusion, by classifying windmill irrigation as a type of lift irrigation, it can be said that between the use of manpower to lift water and the use of the machine power to lift water came a third phase the use of natural power in the form of the windmill.