The Bragg-Brentano focussing method (2θ/θ scan) is widely used in X-ray powder diffractometers. We have recently manufactured a diffractometer (2θ/α scan) whose incidence and reflection sides are asymmetrical. This report describes the optical system, including evaluation and application. The present method enables the following benefits to be obtained by simply changing the sample-to-receiving slit distance b without changing the X-ray source-to-sample distance a: (1) enhanced angular resolution, (2) increased diffracted X-ray intensity, and (3) variable asymmetrical angle. The experimental results show that the focussing condition of this optical system was satisfied. It also shows that the intensity of diffracted X-rays could be increased by approximately 1.4 times at b = 100 mm when the intensity at a = 185 mm and b = 185 mm was assumed to be 1. Furthermore, it was verified that FWHM can be reduced by making sample to receiving slit distance greater.