A system diagnosing method, comprising the steps of: performing a trap operation diagnosis for diagnosing operational conditions of a plurality of evaluation target steam traps in an evaluation target system; calculating, based on the result of the trap operation diagnosis, a total trap-passed steam loss amount (Qt") obtained by aggregating trap-passed steam loss amounts for all the evaluation target steam traps; and generating comprehensive evaluation data, based on the calculation result, characterized in that the method further comprises the steps of: performing a steam leakage diagnosis for diagnosing leakage of steam from evaluation target steam piping in the evaluation target system; calculating, based on the result of the steam leakage diagnosis inputted to inputting means, a total steam leakage loss amount (Qs) obtained by aggregating steam leakage loss amounts by leakage from the piping for the entire evaluation target steam piping; and calculating a sum total steam loss amount (Qts) obtained by adding together the total steam leakage loss amount (Qs) and the total trap-passed steam loss amount (Qt"); and generating, based on the calculation result, as part of the comprehensive evaluation data, data showing a ratio of the sum total steam loss amount (Qts) relative to a total unknown steam amount (Qx) which is a difference between a total receiving steam amount (Qi) and a total necessary steam amount (Qo), as an improvable unknown steam ratio (Kts), in the evaluation target system. performing a trap operation diagnosis for diagnosing operational conditions of a plurality of evaluation target steam traps in an evaluation target system; calculating, based on the result of the trap operation diagnosis, a total trap-passed steam loss amount (Qt") obtained by aggregating trap-passed steam loss amounts for all the evaluation target steam traps; and generating comprehensive evaluation data, based on the calculation result, characterized in that the method further comprises the steps of: performing a steam leakage diagnosis for diagnosing leakage of steam from evaluation target steam piping in the evaluation target system; calculating, based on the result of the steam leakage diagnosis inputted to inputting means, a total steam leakage loss amount (Qs) obtained by aggregating steam leakage loss amounts by leakage from the piping for the entire evaluation target steam piping; and calculating a sum total steam loss amount (Qts) obtained by adding together the total steam leakage loss amount (Qs) and the total trap-passed steam loss amount (Qt"); and performing a steam leakage diagnosis for diagnosing leakage of steam from evaluation target steam piping in the evaluation target system; calculating, based on the result of the steam leakage diagnosis inputted to inputting means, a total steam leakage loss amount (Qs) obtained by aggregating steam leakage loss amounts by leakage from the piping for the entire evaluation target steam piping; and calculating a sum total steam loss amount (Qts) obtained by adding together the total steam leakage loss amount (Qs) and the total trap-passed steam loss amount (Qt"); and generating, based on the calculation result, as part of the comprehensive evaluation data, data showing a ratio of the sum total steam loss amount (Qts) relative to a total unknown steam amount (Qx) which is a difference between a total receiving steam amount (Qi) and a total necessary steam amount (Qo), as an improvable unknown steam ratio (Kts), in the evaluation target system.