Providing a method for reducing occurrences of a mold shift of a cope and a drag that have been molded by a flaskless molding machine and have been assembled by estimating a cause of a mold shift based on measurements and by taking appropriate measures, and a flaskless molding line that uses the method. The method for reducing occurrences of a mold shift of a cope and a drag (1, 2) that have been molded by a flaskless molding machine (200) and have been assembled comprises a step of measuring specific data at positions where a mold shift may occur during a process for manufacturing or taking out the cope and the drag, and a step of determining if the obtained specific data are within an allowable range.