Apparatus comprising a scanning tunneling microscope (STM) has been developed for the first time for use under a plasma environment. It consists of an STM system and a radio-frequency (r.f.) (430 MHz) plasma generation system housed in a vacuum chamber with a rotary pump. A special amplifier with a resolution of 10 fA. up to 1 kV is employed for tunneling current measurements. To prevent noise current from the plasmal the Pt-Ir probe tip is coated with epoxy grease and insulated glass except around its apex of about 3 mu m using the sol-gel method. Under the conventional glow-plasma condition, the noise current from the plasma was deduced to be less than 0.1 nA during plasma exposure for 1800 s. Using this apparatus, the step structure and atomic image of highly oriented pyrolytic graphite (HOPG) were obtained under r.f. low-pressure (0.3 Torr) air glow plasma.