Propagation of the soft X-ray radiation generated by a small-diameter incoherent source through the capillary plane waveguide which satisfies the multimode condition is studied using the Fresnel-Kirchhoff diffraction theory. The diffraction is manifested by appearance of the diffraction fringes, dark and light bands, in the far-field zone of the capillary output. Such a behaviour of the X-ray radiation is confirmed by our experimental data. The results give explanation for the interference effect recently observed in many experiments on the grazing reflections of X-rays in single and multiple capillary optics.