In the normal use of neutron reflectometry, samples with a film thickness of tens nm to a few hundred nm are mainly targeted. In the special case of samples with thicker films about microns, analysis using Kiessig fringes with low order appearing on the low-q region are needed. But in many cases of front incidence, total reflection occurs in the same low-q region. Kiessig fringes with lower order, those are required for analysis of thick films, did not appear and it causes that thick films with thickness of microns were not subject to measurement. However, in the measurement by the neutron introduced from the substrate side (back incidence), it is possible to reduce the critical momentum transfer qc caused by the difference in scattering length density between the substrate and the vacuum as the incident medium. It becomes possible to measure Kiessig fringes on low-q side which is necessary for analysis of thick films with thicknesses of few micrometers.