To solve a problem wherein a knurl and a channel are processed on the outer shape of an insert nut for preventing the outer shape of the insert nut from being rotated and falling out after it is imbedded in a plastic case and under a condition where it is dismantled from the plastic case, trash of a plastic is stuck on the knurl and the channel and it is necessary to remove the stuck trash of the plastic for reusing the insert nut while the original shape is held.

In method for removing the trash of the plastic stuck on the insert nut the insert nut is immersed into a tank in which a solvent, an oil or a chemical is placed and the trash of the plastic is dissolved to clean the insert nut.