The aim of this paper is to evaluate the recreational benefits loss due to the adventives propagation in Ezu Lake in Kumamoto City. The hypothetical travel cost methods, which is one of the methods to evaluate recreational benefits, is applied. The representative of the adventives in Ezu Lake is water lettuce which can multiply 10 times per 28 days. The present state of the lake is maintained by the adventives removal work. In conclusion, (1) the value of the recreational benefits of Ezu Lake under adventives removal work is about 250 million yen, (2) on the other hand, the benefits decrease to 160 million yen under non removal work, which means benefits loss due to the adventives propagation is 91.9 million yen.