In order to ensure the reliability and safety of equipment installed in process lines, it is important that maintenance and management should make efficient use of machine diagnosis techniques. Machine diagnosis by means of acoustic signals has hitherto been beset with difficulty, but there is now a strong demand that new acoustic type diagnosis equipment (utilizing acoustic signals) be developed. In response to this demand, the authors recently conducted research on diagnosis of machine faults by means of the processing of acoustic signals. In this research they were able to develop new acoustic type machine diagnosis techniques, and, using these techniques, to develop acoustic diagnosis equipment for practical use.