In order to evaluate more accurately the corrosion condition of reinforcing steel in chloride contaminated concrete, it is significant to investigate the error level in the linear polarization resistance measurement caused by IR drop. Concretes with eight levels of chloride ions (ranging from 0% to 2.0% by mass of cement) by adding different amounts of sodium chloride in the mixing water were prepared. Linear polarization measurements with and without IR compensation, were applied to determine the error level. Besides, half-cell potential method was employed to detect the corrosion condition of reinforcing steel. The results indicate that the error level is so low (less than 5.0%) that the IR drop can be negligible when the chloride content is relatively lower (0.6% or less by mass of cement). However, the error level is increased with the increase of chloride content. The IR drop is suggested to be compensated when the chloride content is relatively higher (more than 0.6% by mass of cement). At this time, the onset of active corrosion of reinforcing steel is also found.