It is well known any preventive maintenance method is not always useful in cases where system failures roughly follow the exponential distribution. In order to overcome this weakness, the monitoring maintenance was introduced recently and attracted the attention of the engineers as one of the most effective methods to prevent a system from defects and failures. The present authors, Horigome and I, lately introduced the success ratio of fault diagnosis they investigated the characteristics of the preventive maintenance from the viewpoints of the reliability improvement ratio, the maintenance cost and the maintenance numbers. In this case, it is assumed that the system failure occurs through some malfunction, and follows to an exponential distribution. Then the newly obtained results are discussed. Finally it is shown that they are very useful for analysis or investigation on the maintenance in the field.