Silicon is not included in the essential elements for higher plants. However, rice plants absorb a large amount of silicon, and it is well known that silicon fertilization increases the photosynthetic rate of rice and, consequently, dry matter production. Another effect of silicon on rice is the increase in resistance to diseases and insects. Many researchers have, therefore, proposed the use of silicon fertilizers for rice cultivation; for example, Saigusa et al. (1998) proposed the use of porous hydrate calcium silicate (PS), which is produced as an industrial waste in the manufacturing process of light autoclaved concrete, as a cheap and effective material for supplying silicon to rice plants. Raising healthy rice seedlings is important for improving the yield and quality of rice in Japan, and a high dry weight to plant length ratio is one of the prerequisites. Saigusa et al. (2003) reported that Acidified Porous Hydrate Calcium Silicate (APS), made by the addition of sulfuric acid to the PS, was one of the most feasible materials for improving both silicon nutrition and growth of rice seedling. The objective of this study was to evaluate the effect of the application of APS to nursery bed soil and/or PS to paddy fields on the growth and yield of rice plants. Experiments were conducted at the Field Science Center, Graduate School of Agricultural Science, Tohoku University, Japan, in 2002, 2003 and 2004. Rice seedlings (Oryza sativa L. cv. Hitomebore) were grown in a glasshouse for 32, 34 and 34 days, on 2002, 2003 and 2004 respectively. APS was mixed with the nursery bed soil in two ratios; APS: nursery bed soil ratios were 0: 1 and 1: 3 in APS0 and APS25 plots, respectively. Rice seedlings were transplanted to the paddy fields (Typic Melanudands). PS was applied at two rates of 0 and 150 g m^-2 (PS0 and PS150, respectively) before transplanting. Consequently, four silicon treatments were implemented, namely APS0-PS0, APS25-PS0, APS0-PS150 and APS25-PS150, respectively. Rice seedlings were sampled from each treatment immediately before transplanting. Rice seedlings were separated into shoots and roots, and the dry weights of these samples were measured. Rice plants were sampled from each treatment plot at 14 days after transplanting, and the root length was measured with a root length scanner. At harvest time, rice plants were collected for the measurement of the dry weight, chemical analysis of the silicon content and yield survey. Dry weight of seedling shoot increased due to the APS treatment plot in all years, and the ratio of shoot dry weight to plant length in the APS25 treatment plot was higher than that in the APS0 treatment plot in 2002 and 2004. Silicon contents in the shoots of the seedlings in the APS25 treatment plot were 31 to 63 % higher than those in the APS0 treatment plot. Regardless of the year of experiment, the tiller number of rice plants increased by APS treatment. The total root length in APS treatment plot was longer than that in the APS0 treatment plot in all years. The amounts of silicon uptake by the rice plants were 20.0, 25.9, 30.2 and 36.4 g m^-2 in 2002, and 17.9, 18.4, 21.2 and 25.9 g m^-2 in 2003, and 30.8, 35.6, 46.4 and 43.8 g m^-2 in 2004, in the APS0-PS0, APS25-PS0, APS0-PS150 and APS25-PS150 treatment plots, respectively. In 2002 and 2004, no significant differences in grain yield were found among the treatment plots. On the other hand, in 2003 with cold summer, grain yield in the APS0-PS150 treatment plot was 18 % higher than there in the APS0-PS0 and APS25-PS0 treatment plots (p<0.05, Turkey' s HSD). Grain yield in the APS25-PS150 treatment plot was 36% higher than that in the APS0-PS0 and APS25-PS0 treatment plots, and 13% higher than that in the APS0-PS150 treatment plot. In 2002, the number of panicles in the APS25-PS0, APS0-PS150 and APS25-PS150 treatment plots were 16 - 27 % higher than that in the APS0-PS0 treatment plot, while although there were no significant differences among the treatments with regard to the other yield components. In 2003, the percentages of ripened grains in the PS0 treatment plots were 57 to 63 %, while 82% in the PS150 treatment plots. In 2004, no significant differences were found among the treatment plots in the yield components. From the above results, we concluded that both acidified porous hydrate calcium silicate applied in nursery bed soil and porous hydrate calcium silicate applied in paddy field were effective materials for improving the silicon nutrition and growth of rice plants; its effectiveness was enhanced by the combination treatment.