The single electron transistor (SET) is fabricated using the scanning tunneling microscope (STM) as a fabrication process, and the fabricated SET operates at room temperature. Using the STM tip as a cathode, the surface of the titanium metal can be oxidized, and the few tens of nanometer wide oxidized titanium line can be made. The small island region of the SET of ∼30×∼35 nm2 is formed by the oxidized titanium line. The Coulomb staircase of 150 mV period is observed in the current–voltage characteristics of the SET at room temperature.