We have demonstrated subnanometric stabilization of tip-enhanced optical microscopy under ambient condition. Time-dependent thermal drift of a plasmonic metallic tip was optically sensed at subnanometer scale, and was compensated in real-time. In addition, mechanically induced displacement of the tip, which usually occurs when the amount of tip-applied force varies, was also compensated in situ. The stabilization of tip-enhanced optical microscopy enables us to perform long-time and robust measurement without any degradation of optical signal, resulting in true nanometric optical imaging with high reproducibility and high precision. Even though small-sized samples such as carbon nanotubes and DNA nanocrystals with sizes smaller than 10 nm were positioned under the silver tip, they did not affect the sensitivity and the accuracy of our drift compensation method. This is because scattering efficiency of these samples is much smaller than the metallic tip with the apex size of 40 nm. The technique presented is applicable for AFM-based nanoindentation with subnanometric precision.