[en] Digital hammering inspection system with AE (Acoustic Emission) sensor has been developed and the effects of state, stiffness, etc. on the natural frequency of metal and concrete objects to be inspected were evaluated in terms of improper installations and age-related degradations. The system has been widely applied to social infrastructural diagnosis including expressways (West Nippon Expressway Company Limited, NEXCO-WEST), nuclear power plants (ex. Chubu Electric Power Co., Inc.), reprocessing plant of spent nuclear fuel (Japan Nuclear Fuel Limited) and a successful on-site trial of a new technology of i-Construction led by MLIT (the Ministry of Land, Infrastructure, Transport and Tourism). (author)