A diffraction mechanism, and an optical model to reflect that mechanism, for a light-diffusing film with an alternate-polymer-layer structure was proposed and validated. According to this model, the film forms an angular distribution of light intensity that is almost constant in a certain scattering-angle (cutoff angle) range and drastically decreases outside that range; that is, the profile is similar to a trapezoid. Although the trapezoid intensity distribution (TID) is a fundamental distribution of the film, the mechanism to form the TID has not previously been clarified. A key mechanism is that the refractive-index distribution of the layer structure is expressed as stacked phase gratings, some of which should diffract zeroth-order light to higher orders strongly, and the repetition of this multiple diffraction spreads light over the whole cutoff angle range, thereby generating the TID. To verify the proposed mechanism, intensity distributions were calculated by using the proposed model. The calculation results indicate that the proposed model reproduces the TID.