An atomically Rat and clean surface of a 0.05 wt.% Nb-doped SrTiO3(100) sample was obtained by exposure to radio-frequency (430 MHz) glow plasma. The surfaces were studied by scanning tunneling microscopy/spectroscopy (STM/STS) after exposure to cold plasma to obtain information on the surface structure ex-situ plasma environment. The density of states (DOS) was estimated from STS data by the dl/dV scheme and its extension method. After 15 min of air plasma treatment, the root-mean-squares (rms) roughness of the surface, which covered an area of 500 nm(2), had decreased from 1.60 to 0.46 nm. Five types of DOS were deduced from STS data before and after treatment. Four of these were obtained only after these plasma treatments.