We examined the resolution of optical images of scanning near-field optical/atomic force microscope (SNOAM) by synchronizing the modulated light to be probe vibration. The system examined in this paper, which we have developed previously, uses a sharpened and bent optical fiber probe. The probe was vibrated vertically toward the specimen stage by dynamic-mode atomic force microscopy function. We controlled the phase and the range of the modulated light using a phase shifter to change the irradiation position. The resolution of optical images showed remarkable change over a 40 nm vertical range of the irradiation position from the sample surface. We obtained the best resolution of the optical image by optimizing the phase and the range of the irradiation. The aperture size was estimated to be 55 nm by comparing the best optical image and topography of our standard pattern.