PROBLEM TO BE SOLVED: To enable the prevention of propagation of pests such as sundry germs at the earth floor inside a clean green house, the avoidance of the influence of outer weather and the obtaining of stable harvest the year round. SOLUTION: The earth floor inside a clean green house P is covered with a waterproof sheet 5, and a main duct 7 for taking-in outer air having an introducing fan 6 and plural clean air-supplying branch ducts 8 which are all connected to the main ducts through a filter 9 for filtering the outer air introduced into the main duct by the introducing fan 6 are placed inside the clean green house P. The branch ducts 8 are each extended and placed below a culturing circulating apparatus 2 of a multi-staged bench structure, which is placed inside the clean green house P, for placing culturing containers Q. The culturing circulating apparatus 2 comprises a lower stage pool 3B for the seedling raising period, which moves the culturing container Q to the downstream side with the flow due to buoyancy by pouring water into the culturing circulating apparatus 2 and bringing it into a water filled state, and an upper stage pool 3A, which is placed above the lower stage pool 3B. The upper stage pool 3A moves the culturing container Q, whose raising of seedlings is completed, to the upstream side from the downstream side. When the culturing container Q flows and reaches the downstream side, it is kept there until the plants grow up to the state they are ready for harvesting.
