An abrasive has a plate shape with a flat surface, in which a maximum diameter of the flat surface thereof is in the range of 0.05 mm to 10 mm, and 1.5 to 100 times as the maximum diameter as thick of the abrasive, and the blast processing method is one in which this abrasive is ejected by being inclined at an incident angle with respect to a surface of a product to be treated. The ejected plate-shaped abrasive slides along the surface of the product to be treated while having the flat surface in slidable contact with the surface of the product to be treated which is an object surface to be treated, so that the surface of the product to be treated is flattened by removing the peaks only, without increasing the depth of the valleys of the roughness curve.