A laser processing device includes a beam splitter disposed between a focusing lens and a protective window, a return light measurement unit configured to measure intensity distribution of a return light reflected from a workpiece and returning to an external optical system via the beam splitter, a storage unit configured to store at least one of normal pattern data representing the intensity distribution of the return light when the protective window is in normal condition and abnormal pattern data representing the intensity distribution of the return light when the protective window is contaminated, a processing unit configured to perform a process of detecting contamination of the protective window during laser processing based on measurement data about the return light and at least one of the normal pattern data and the abnormal pattern data, and a warning unit configured to warn of contamination of the protective window in accordance with the process.