An object of the present invention is to stably yield a leafy vegetable having a low potassium content even when large scale hydroponics is practiced. The cultivation is carried out by strictly controlling the concentrations of the hydroponic solutions based on the EC value throughout the entire cultivation period as well as by changing the N:P ratio between the potassium containing hydroponic solution used in the first part of the cultivation period and the subsequently used potassium-free hydroponic solution. As a result, leafy vegetables having a potassium content of about ¼ of that of the typical hydroponics leafy vegetables can be yielded with a stable quality even in large scale hydroponics.