The invention provides a method of diagnosing a system, a method of operating an aggregating system for system diagnosis and an aggregating system for system diagnosis, which are effective for achieving comprehensive and effective system cost reduction. For a target system, at least two or more kinds of diagnoses selected from a trap operation diagnosis, a fluid leakage diagnosis, a system improvement diagnosis and maintenance improvement diagnosis are effected in a batch and in reporting the results of these diagnoses in a batch, there are reported an economic advantage MQt'' obtained by reduction in a trap-passed seam loss Qt'', an economic advantage MQs, MQp, MQn obtained by reduction in fluid leakage loss Qs, Qp, Qn, an economic advantage Ma obtained by system improvement and an economic advantage Mb obtained by method improvement in a maintenance method.