Allowing for secondary extinction (SE), a method is described for thickness measurement of thin films by x-ray absorption. The method is tested by employing Al foils and electrodeposited Cu films detached by the substrate. The thicknesses determined by the method were in fair agreement with that ones measured by the ordinary absorption method based on using incident beam intensities. Moreover, a dependence of the SE coefficient on the transmission factor of the films was experimentally shown, and thus additional light was thrown on the nature of SE.