A descaling method for projecting a projection material using a blast device and descaling a processing object made of an iron-based material, wherein the descaling method is provided with: a projection material loading step for loading the blast device with an unused projection material having a Vickers hardness of HV300-600; a particle diameter distribution adjustment step for operating the blast device to adjust the particle diameter distribution of the projection material in the blast device to a predetermined particle diameter distribution; and a descaling step for projecting the projection material that has been subjected to the particle diameter distribution adjustment step onto the surface of the processing object. When the particle diameter distribution of the projection material that has been subjected to the particle diameter distribution adjustment step is divided into first granules having a particle diameter exceeding 300 [mu]m, second granules having a particle diameter of up to 300 [mu]m and exceeding 75 [mu]m, and third granules having a particle diameter no greater than 75 [mu]m, the relationship (ratio of the second granules) >= (ratio of the first granules) >= (ratio of the third granules) obtains.