The effect attributable to the nonparallelism of the guiding layer of a planar waveguide on the reflection dip of the attenuated total reflection (ATR) spectrum is investigated. It is considered that the reflected light from a nonparallel waveguide is, indeed, the superposition of a series of beams propagating in different directions. The ATR spectra are numerically calculated with various inclination values. Compared with those of the corresponding parallel ones, with the inclination of waveguides exceeding 1 μrad, the characteristics of the ATR spectra begin to differ considerably, and when the inclination reaches 10 μrad, the sensitivity of sensors based on the waveguide is reduced by 62.7\% and 67.3\% for the free space coupling structure and the prism coupling structure, respectively. When the inclination becomes larger than 100\mathrm{\mu rad} , the ATR phenomenon cannot be observed any longer.