This study treats the internal friction angle of the slope failure materials in motion. The authors have first investigated the relation between the void ratio and the coefficient of friction angle by various direct shear tests with Toyoura standard sand. They have next examined the friction angle in the residual condition with the new designed ring shear apparatus with some samples. The results are as follows: 1. In a low void ratio there are many differences in the relation between the void ratio and the coefficient of friction angle of various shear types, but near the residual condition there are almost no differences between them. 2. The friction angle in motion is the same value as that of the residual condition, regardless of the shear velocity. It means that a general low of physical friction applies for granular materials, too. 3. The friction angle in the residual condition can be measured with the conventional direct shear apparatus, beginning with the test in the loose sample condition. The value of the tests can be used for the f. iction angle of slope failure materials in motion.