Corrosion of steel bars in underground RC structures due to inflow of groundwater containing salt has been reported. Since underground structures are sometimes subjected to external loads such as ground subsidence, it is becoming increasingly important to predict the superimposed effects of salt-induced deterioration. In this study, loading tests were conducted with 80% of a steel-corroded RC hollow circular specimen buried underground to investigate the effects of external loads under underground restraint. The results showed that the deformation capacity and bearing capacity of the underground circular tunnel structure were not significantly affected by steel corrosion, but shear failure through the cross-section along the leading bending cracks sometimes occurred. This shear failure can occur outside of the corroded zone if there is asymmetry in the corroded zone of the rebar. Furthermore, the shear strength of the subsequent shear failure along the bending crack may be lower than the preceding shear strength equation, which should be taken into account in maintenance.