We study resonant frequencies of water sessile drops in various oils. The resonant frequency is experimentally obtained as the frequency shift of the scattered laser light, using an optical heterodyne technique. The resonant frequency is explained by the capillary wave propagating along the meridian curve of the drop with the fixed-end boundary condition at the three-phase contact line, although the sessile drop in air considerably deviates from the fixed-end boundary condition. We study this point further by observing the vibration directly with a fast video camera, and by analyzing the contour of the snapshot near the contact line. The results agree with the resonant frequency analysis results, where the number of half waves in the meridian curve of the water sessile drop is an integer when the drop is in oil, while in air, it deviates considerably from an integer.