To clarify the physiological role of SiO_2 in the growth of rice plant, characteristics of leaf gas exchange and dry matter production were investigated using four rice varieties (Houyoku, Habiganj, Akenohoshi, Blue belle) grown in culture solutions. SiO_2 concentration in the culture solution were set at 0, 10, 30 and 100 ppm and plant were grown under greenhouse conditions. Main results obtained are summarized as follows; 1. Plants grown without SiO_2 had soft and willowy leaves, whereas those at high SiO_2 had more erect leaves and better light-intercepting characteristics. 2. Growth of the plant treated with high SiO_2 concentration was better than those plant without SiO_2. But there were some varietal differences in the response to SiO_2. 3. Numbers of tillers and leaves were increased with increasing SiO_2 content in Akenohoshi and Blue belle. Consequently, leaf area per plant increased. On the other hand, plant length was increased with increasing SiO_2 content in Habiganj. No changes in growth characteristics were found in Houyoku. 4. Effect of SiO_2 on rate of CO_2 assimilation and transpiration and on leaf conductance were not apparent in the rice varieties used. The hypothesis that increment of SiO_2 regulate the cuticle transpiration and window theory were not demonstrated.