Green-house tomato (Lycopersicon esculentum Mill.) was grown on summer-autumn harvest culture to examine a diagnostic technique for nitrogen nutrition. 1. Lower leaves were considered to indicate nitrogen nutrition of tomato plant. In lower leaves, petiole sap nitrate concentration was higher than that of higher leaves, and correlated with nitrogen application rates. The leaf petiole just below the first truss was chosen as the best section to sample for diagnosis of nitrogen nutrition. 2. In 2000, fruit yield and nitrogen absorption were higher than those in 1999, and petiole sap nitrate concentration was lower than that in 1999. It was thought nitrate absorbed from roots was assimilated immediately and nitrate accumulation in petiole was reduced. It was further thought that the difference in light conditions between 1999 and 2000 influenced nitrogen absorption and petiole sap nitrate concentration in each year. 3. In each year fruit yield was maximized when petiole sap nitrate concentration was about 6000 mg L^<-1> and decreased when lower than 4000 mg L^<-1>. 4. As petiole sap nitrate concentration increased, nitrogen absorption/application rate decreased and nitrate nitrogen concentration of cultured soil increased. Petiole sap nitrate concentration was considered to indicate nitrogen recovery rate. 5. As the results, the best sampling section was the leaf petiole just below the first truss. To consider fruit yield, nitrogen absorption/application rate and soil nitrate nitrogen concentration, the proper range of petiole sap nitrate concentration was 4000-7000 mg L^<-1>.