We analyzed the genotype of the rice variety 'Norin-PL8', which harbors the genes for cool tolerance at the booting stage derived from a highly cool-tolerant javanica rice variety, Silewah. As a result, we detected the chromosomal segments introgressed from Silewah into Norin-PL8. Furthermore, we analyzed the quantitative trait loci (QTLS) for cool tolerance using RFLP markers on the introgressed segments. The results indicated that at least the introgressed segments on chromosomes 3 and 4 are responsible for the cool tolerance.