To provide a simple method for detecting a malfunction of steam trap.

A temperature sensor 12 on a steam side to detect a temperature of a special portion Xs in a case on the steam side or a temperature ts of a case section 3s positioned opposite to the special portion Xs in the case on the steam side and a temperature sensor 13 on a drain side to detect a temperature of a special portion Xw in a case on the drain side or a temperature tw of a case section 3w positioned opposite to the special portion Xw in the case on the drain side are provided to indicate these detected temperatures ts, tw or to indicate a detected temperature difference <t or a diagnostic means 14 is provided to execute a desired corrective measure against an error when the detected temperature difference <t becomes smaller than a set threshold value <ts.