The rotated-inclined double-crystal monochromator (RIDCM) has been adopted to reduce the heat load from third-generation undulator radiation. The position of the exit X-rays from RIDCM has been calculated as a function of X-ray energy on the basis of diffraction theory including refraction effects. The results show that the positions of the exit X-rays vary over a wide range due to asymmetric reflection. Methods of fixing the exit position in RIDCM are also discussed.