To provide a method suitable for detecting a deviation between upper and lower dies in a flaskless horizontally split mold.

The distance between one side of the upper or lower dies and a first distance measuring means is respectively measured by the first distance measuring means, the differential distance between the respective one side of the upper and lower dies and the first distance measuring means is operated based on the result of measurement, the distance between the other surface of the upper and lower dies and a second distance measuring means is respectively measured by the second distance measuring means, and the differential distance between the respective other side of the upper and lower dies and the second distance measuring means is operated based on the result of measurement.