PURPOSE: To provide a detection method for sand blockage of an oil hole in a gear carrier capable of easily detecting mechanically the existence of internal sand having remained inside of oil holes of such a casting as gear carrier even if the shape of remaining internal sand is not identified.

CONSTITUTION: In the detection method for internal sand having remained inside of oil holes 1 and the like of a casting such as gear carrier, a thrust rod 3 made of resin with a smaller dimension than the oil hole 1 in a shape similar to the oil hole 1 is provided to a robot hand via a piezoelement 4 and the existence of internal sand is detected from the abnormal load exhibited in the output voltage change generated in the piezoelement 4.