A triaxial compression apparatus having simple mechanism was constructed by the authors in order to investigate the failure condition of rock-like materials under the stress state in which the values of three principal stresses differ from one another. Triaxial compression tests applying uniform confining pressure and biaxial compression tests were performed using three types of rock and cement mortar. Two failure curves obtained by the triaxial test showed different feature, one of which shows precipitous decrease in region where ratio of two principal stresses σ1 and σ2 approaches to unity, while the other did not show such a feature. Such difference may be due to difference in magnitude and direction of maximum tensile stresses along the periphery of minute three dimensional voids which are supposed to pervade the material.