The present invention relates to a circular nutrient solution supply apparatus, which is configured to supply a nutrient solution necessary for a nutrient solution bed used in a nutrient solution culture of plants, to collect the nutrient solution discharged from the nutrient solution bed, to process so as to be reused and to supply to the nutrient solution bed again. The circular nutrient solution supply apparatus comprises a collected nutrient solution tank connected to the nutrient solution bed, and collecting and storing the nutrient solution discharged from the nutrient solution bed； a collected nutrient solution processing unit connected to the collected nutrient solution tank, and sterilizing and purifying the nutrient solution supplied from the collected nutrient solution tank； a purified nutrient solution tank connected to the collected nutrient solution processing unit and storing the sterilized and purified nutrient solution supplied from the collected nutrient solution processing unit； a multi-way valve formed on a pipe re-supplying the nutrient solution from the purified nutrient solution tank to the nutrient solution bed； a source water tank connected to the multi-way valve, and storing the source water supplied to the pipe re-supplying the nutrient solution from the purified nutrient solution tank to the nutrient solution bed； a liquefied fertilizer supply unit connected to the multi-way valve, and supplying liquefied fertilizer supplied to the pipe re-supplying the nutrient solution from the purified nutrient solution tank to the nutrient solution bed； a first sensor unit measuring the electric conductivity of the nutrient solution discharged after being sterilized and purified at the collected nutrient solution processing unit； and an EC control unit controlling the amount of the source water supplied from the source water tank and the liquefied fertilizer supplied from the liquefied fertilizer supply unit by controlling the multi-way valve based on
