A method for an X-ray stress measurement can be classified into the method to determine the orientation angles for measuring the diffraction peak and the method to determine the optimum stress values. In this study, a new method for an X-ray stress measurement using an area detector is proposed and is named ”direct method”. In the direct method, the diffraction ring due to the incident X-ray beam normal to the specimen surface is used. The optimum stress values are determined by a simplex method as a kind of linear programming. The direct method was applied to the measurement of the residual stresses in an austenitic stainless steel, SUS316L, to examine its effectiveness. The water-jet peened residual stresses were measured by the direct method, and the obtained stress values were close to one obtained by the other method. In addition, the direct method was applied to measuring the ground residual stresses. After correcting the center of the diffraction ring, the stress values optimized by the simplex method corresponded with one measured by the sin2 y method.