PURPOSE: To provide an oxygen partial pressure control type scanning tunneling microscope which can observe an oxidation reduction of a sample while controlling this reaction.

CONSTITUTION: An oxygen partial pressure control type scanning tunnel microscope includes a gas supply means 1 for supplying gas so that a sample is observed in a gaseous environment, an oxygen detection means 4 for detecting the oxygen partial pressure in the gas, and oxygen partial pressure control means 3, 8 for controlling the oxygen partial pressure in the gas, and the oxygen partial pressure is controlled according to whether oxidation or reduction is carried out according to an equilibrium oxygen partial pressure value determined by the sample for observation and by the temperature of the sample. Therefore, the oxygen partial pressure value is controlled about the equilibrium oxygen partial pressure value and the process of a high-temperature oxidation reduction of the sample can be observed using the scanning tunneling microscope.