Recently the phenomena have happened in Japan that spalls of tunnel lining concrete fall from its surface. Therefore the method for diagnosing interior defects of tunnel lining concrete has been desired to develop. Several diagnostic methods have been proposed, but most of them are in the test stage and don't come to practical use. In this paper, we describe the development of the diagnostic device with low frequency elastic wave and the results of the tests by this device in real tunnel to verify the method for diagnosing existence and depth of interior defects of tunnel lining concrete. We also describe the results of the tests by this device for concrete models and of dynamic numerical analysis.