To develop a technique for efficiently managing fertilizer application for cucumbers in protected cultivation, real-time diagnosis of the soil solution was examined by using simple instruments to obtain the nitrate content of the soil solution as an indicator. As a result, since the value of the nitrate content measured in the soil solution using a compact nitrate ionmeter was comparable to the value measured by the phenolsulfuric acid method, it became possible to use a compact nitrate ion-meter to measure the nitrate content of the soil solution in the field. Also, based on the relation between the nitrate content of the soil solution and the yield of cucumber fruit, to secure stable production, the nitrate content in the soil solution should range from 400 to 800 mg･L⁻¹ during the harvest period for semi-forcing and retarding cultures.