Variation of rock strength has an important role in designing the various rock structures. Though strength in uniaxial stress state such as uniaxial compressive strength has been well investigated up to now, the strength in complicated stress state such as triaxial compressive or shear strength has been studied only to very limited extent because of its difficulty in experimental works. For example, variation of shear strength should be studied under various normal stresses and then many specimens are necessary. In this study, direct shear tests of red shale were conducted under the normal stress from 1 to 6 MPa. A special care was taken to make as many specimens as possible from the limited sample rock, and total number of 397 specimens were tested. It was found that the variation of Δτ, which is the difference between the shear strengths measured in testing and calculated with the regression line, did not depend on normal stress. The obtained failure criterion and variation of strength for the red shale were discussed comparing the envelope of rock stress which was reported previously and the results of other rocks in various testing conditions.