The reflected optical second harmonic generation from the interface between Ge (111) and germanium oxide layers has been measured in dry nitrogen atmosphere as a function of the sample rotation angle around its surface normal. The germanium oxide layer was produced either by oxidizing the Ge (111) substrate in air at room temperature (natural oxidation) or by oxidizing the Ge (111) substrate in pure oxygen gas at 823 K (thermal oxidation). A difference was found in the SH intensity in p-in/p-out polarization configuration between the Ge/native-oxide interface and the Ge/ thermal-oxide interface. By XPS measurement, we found that GeO and GeO2 were formed at the Ge (111) /nativeoxide interface and at the Ge (111) /thermal-oxide interface, respectively. We conclude that the difference in the interface structure has led to the difference in the SH intensity pattern.