There are a lot of street lights in Japan. Some of these street light are damaged or corroded by water or condensation in the winter. In the worst case, a street light falls down to injure person or damage a car and so on. To prevent these accidents, periodical inspection is required. Most common inspection methods are visual inspection or thickness inspection by ultrasonic sensor. Visual inspection has following problems, i.e., (1)quality of inspection depends on a inspector, (2)it takes a lot of cost and time to inspect a street light. The thickness inspection by ultrasonic sensor has problems of poor contact of sensor to a street light especially at uneven surface caused by corrosion. In this paper, we excited a street light to measure its vibration and then estimated reduction of thickness of pole for several street lights in Osaka prefecture and checked its performance. Finally, good ability of damage detection was confirmed.