The development of rice root system has been studied in Japonica-Indica hybrid cultivars (J-IH), Milyang 23 and Suweon 264, and Japonica type cultivars (JT), Nipponbare and Koshihikari, grown in a paddy field. Soil cores were taken below (and between) hill(s) at the panicle initiation, the heading and the grain-filling stages. After the soil cores were divided into layers by soil depth and the roots were washed out, length and weight of the roots in each soillayer were measured to calculate root length density (RLD) and root weight density (RWD). At the panicle initiation stage, there were no significant differences in RLD and RWD in the 0-25 cm soil layer below hill of each cultivar. Thereafter, RLD and RWD markedly increased, especially in J-IH, and at the grain-filling stage RLD and RWD of J-IH both below and between hills were significantly larger than those of JT. Such increases of RLD and RWD in J-IH after the panicle initiation stage occurred mainly in shallow soil layers. The soil depth of the maximum value in RLD and RWD below hill were different from each other ; the 0-5 cm soil layer in RLD and the 5-10 cm soillayer in RWD. The specific root length in 5-10 cm soillayer below hill was smaller than those in the other soil layers below and between hills, especially in the later growth stage, which might depend on the distribution of root diameter and branching.