Tomato plants were grown using a rockwool-based drip culture, and the nitrous oxide (N2O) emission from the rhizosphere was precisely observed using an automatic monitoring system. Fertilization was intermittently conducted four times daily for 1 h each time (the standard conditions). Basically, N2O emissions were very quick and exact responses against the fertilization. Under the standard conditions, N2O emissions started within 30 min and peaked within 90 min from the beginning of each fertilization, and more than 90% of all emissions occurred within the 2 h following fertilization. The number of peaks and duration for N2O emissions altered synchronously with the fertilization. On the other hand, the maximum rate and the total amount of N2O emissions did not always reflect the fertilization quantity. For example, halving the concentration of the fertilizing solution decreased both the maximum rate and total amount of N2O emissions to approximately one-fifth ofthat observed under the standard conditions. The observed N2O emissions varied 1.0-4.6% of applied nitrogen among three trials under the standard conditions. It is possible that another factor, such as crop size and vigor, affect the emission. The monitoring would allow us to know the optimum timing and quantity of fertilization.