The invention provides a method for detecting laminated iron cores. Laminated cores are formed by laminating a plurality of iron chips having a predetermined shape and includes a cooling flow path for allowing refrigerant to circulate, the refrigerant is supplied and discharged by forming openings at different locations and the method includes supplying and discharging the method :the light-projecting portion and the light-receiving portion of the photosensitive sensor are respectively arranged at the opening of the cooling flow path, and detecting light from the light projection unit with the light receiving section., thus, the through-state of the cooling flow path is detected.