In this study, a simple thickness evaluation method was proposed for corroded road-lighting poles based on thickness measurement results of a dismantled road-lighting pole which had been used for about 40 years. The remaining thickness in cross-section of this specimen was measured by using both of Ultrasonic Thickness Gauge (UTG) and 3D portable laser scanner in order to investigate the accuracy of average thickness which could be obtained from only 4 measuring points at intervals of 90 degrees in circular cross-section. In this UTG measurement, the smoothing process, which will be carried out ordinarily by using electric grinder around measuring points, was abbreviated from the considering of labor-saving and additional thickness loss. From the measurement results, UTG measurement under the condition of no smoothing process will overestimate the actual remaining average thickness depending on surface roughness due to corrosion. Finally, an example of simple thickness evaluation, which uses average thickness and standard deviation based on only four measuring points, was shown for more reasonable UTG inspection of corroded road-lighting poles.