PROBLEM TO BE SOLVED: To provide a trouble diagnosing method and device for servo valves under hydraulic pressure that can more accurately determine the presence or absence of trouble in servo valves under hydraulic pressure and reduce the occurrence of misjudgment.SOLUTION: A trouble diagnosing method for servo valves under hydraulic pressure that automatically diagnoses any trouble in servo valves under hydraulic pressure comprises a signal inputting step of inputting a signals indicating the active/inactive state of a pump supplying hydraulic oil and signals indicating the open/closed states of multiple electromagnetic valves, a rolling mill operating state determining step of determining the operating state of a rolling mill on the basis of the signal inputs, a threshold selecting step of selecting a threshold on the basis of the determined operating state, and a trouble diagnosing step of diagnosing the presence or absence of trouble in the servo valves under hydraulic pressure by comparing the selected
