The present study was undertaken, using children attending special classes for mentally retarded as subjects, in order to clarify the effects of physical activities on their cognitive abilities. All the subjects were tested and re-tested with two months of interval in motor ability, intelligence and discrimination ability of directions. The subjects in the experimental group (mean IQ by WISC, 600) were engaged with 45-minute long daily physical excercises during the experimental term, aiming at improvement of balancing and rhythmic abilities, combined with various ball-games, while those in the control group (mean IQ by WISC, 58.7) participated in the regular curriculum. As the result, the balancing and rhythmic abilities among the experimental group subjects were significantly improved, and hence the effect of prescribed exercises was recognized. The ability of left-right discrimination among the experimental subjects were found to be significantly larger in its improvement (left-right discrimination of body segments, P<0.01), while no significant differences were, observed between the experimental and control groups in the amount of changes in intelligence, shown as IQ score improvements, though IQ scores were greater in the re-test in both groups.