We developed a method of detecting a void under concrete plates using attenuation rate of longitudinal elastic wave in impact-echo test. When a concrete plate contacts with the soil under the plate, a part of energy of longitudinal elastic wave is transmitted into the soil, and the longitudinal elastic wave attenuates rapidly. On the other hand, when a void exists under the plate, a longitudinal elastic wave is reflected completely from the undersurface of plate, and the longitudinal elastic wave attenuates slowly. Then by using the attenuation rates of longitudinal elastic wave, we can distinguish whether a void exists or not under concrete plates. We performed experiments to detect a void under a concrete plate. The surface of the concrete plate with the thickness of 150 mm. was impacted by a steel ball with the diameter of 10 mm. The vibration of the surface was measured with Laser Doppler vibro meter. We measured the attenuation rate for the case when the concrete plate contacted with the soil under the plate and the case when a void existed under the plate, respectively. We could distinguish whether a void exists or not by using the attenuation, rate.