A purpose of the present invention is to provide a recirculating hydroponics system which reuses a nutrient solution, capable of minimizing the scale of the system even without the reduction of a nutrient-water utilization efficiency due to a coercive discharge of a drainage water storage amount exceeding the scale of the system even in a commercial scale. The present invention relates to a recirculating hydroponics system. A recirculating hydroponics system (10) reusing drainage water according to the present invention includes: a setting unit which sets irrigation intervals and nutrient solution parameters； a first memory device which stores supply amount data and electrical conductivity (EC) value data of the nutrient solution, calculated by the irrigation intervals and nutrient solution parameters； a first EC measuring instrument which measures EC values of drainage water discharged from a cultivation bed； a second memory device which stores the EC values measured by the first EC measuring instrument； a nutrient solution mixing tank which mixes a nutrient solution required in crops growing in the cultivation bed, and stores the mixed nutrient solution； a raw water supply tank which supplies raw water to the nutrient solution mixing tank； a drainage water collecting tank which collects the drainage water discharged from the cultivation bed； a drainage water storage tank which stores the drainage water transferred from the drainage water collecting tank, and supplies the drainage water to the nutrient solution mixing tank； a concentrated nutrient solution storage tank which supplies a concentrated nutrient solution to the nutrient solution mixing tank； and a central control unit which calculates an amount of the drainage water supplied from the drainage water storage tank to the nutrient solution mixing tank, an amount of the raw water supplied from the supply tank to the nutrient solution mixing tank, and an amount of the concentrated nutrient solution supplied f
