We discuss the detection of a single spin by monitoring the tunneling current of the scanning tunneling microscope in the presence of a magnetic field. We have measured high frequency signals in the tunneling current of scanning tunneling microscopy for a submonolayer oxide thin film on the Si(111)-7×7 surface. We demonstrate that the signal is related to the Larmor precession of the electron spin associated with a dangling bond. The detected precession frequency possesses a broad distribution (line-width is comparable with that observed by conventional electron spin resonance) and a split near the maxima, both of which are attributed to the inhomogeneity of the g factor of a single spin. With site-specific measurement, we found that the spin signal was detected on the bright Si adatom in which oxygen atoms occupy the backbonds and weakened the metallic nature of the Si(111)-7×7 surface. The measured Larmor frequency corresponded to g∼2.00. On the other hand, no peak was detected on the dark adatom that is tied with an oxygen atom at the on-top site. This is due to the disappearance of the Si dangling bond through the chemical bond formation with the oxygen atom.