Resonance characteristics of whispering gallery modes on a dielectric disk coupled with a straight waveguide are investigated theoretically and experimentally. Taking account of a phase perturbation caused by a coupling phenomenon, we analyze the resonance characteristics of the disk resonator with the straight waveguide. The straight waveguide is placed near the disk to excite the whispering gallery modes or to detect an output power from the disk. In a region where a gap between the straight waveguide and the disk is small, a phase distribution of an electromagnetic field is perturbed by the coupling phenomenon. The coupling phenomenon is analyzed by a coupled mode equation for nonparallel dielectric waveguides. As the result, a discrepancy of resonance frequency intervals (free spectral range) between the analytical results and experimental ones is several megahertz in a 24-GHz band. It is also found that a relative error between the analytical and experimental resonance frequencies is about 1.3%. The analytical method and the results will be applied to the precise design of a high Q resonator with an external circuit in millimeter to optical range.