A problem diagnosis method and problem repair method for a laser device are provided. The method includes measuring the intensity of scattered light generated by an optical part inside the laser device; referring to data indicating a temporal change in the scattered light predicted under predetermined operating conditions of the laser device, and predicting the seriousness of the problem from the intensity of scattered light; and determining what kind of maintenance work is necessary based on the seriousness of the problem. Additionally, the seriousness of the problem in the optical part can be predicted by executing fuzzy logic based on membership functions defining the relationship between scattered light intensity and the seriousness of problems of optical parts.