PROBLEM TO BE SOLVED: To provide a method for using a fallow paddy field, which enables the use of the fallow paddy field in the state of a paddy field, the maintenance of the functions of the paddy field, and the production of products having high additional values. SOLUTION: This method for using a fallow paddy field, comprising culturing a micro-alga useful as a raw material for foods or medicines, culturing a micro- alga useful as a grass-inhibiting agent or fertilizer for paddy fields, or culturing a micro-alga useful as a bait for fishes or shellfishes and simultaneously breeding the fishes or shellfishes. Thereby, the products having high additional values can be produced using the fallow paddy field, and the employment of agrochemicals and chemical fertilizers in the paddy field can be reduced. Since the fallow paddy field is used in the state of the paddy field, the original water- storing function and ecosystem-maintaining function of the paddy field and the spectacle of the paddy field are not deteriorated, and the recovery of the fallow paddy field is also easy.
