The relationship between mass or distribution of rice roots and yield were studied. Materials were rice plants cultivated in an experimental paddy field under various conditions in relation to the amount of nitrogen fertilizer and planting density. Soil cores were taken at the site below and between hills at a 30cm depth with a metallic cylinder (diameter : 15cm, length : 40cm) . After the roots were washed out using water on a sieve, the length of all the roots were measured with the Comair root length scanner made by Commonwealth Aircraft Corp., Ltd. From these data the root length density at each site and depth was calculated. With these root length densities the total root length per unit area (depth: 30cm) was estimated. The total root length increased with yield up to about 550g/m2 but not very much over 600g/m2. Then the distribution of roots were analyzed using the root length density. At the yield level over about 600g/m2 the proportion of deeper distributed roots was higher. These results indicate that the deeper distribution of roots was important for high rice yield.