A method for polishing a diamond crystal, the method comprising: preparing a diamond crystal having a main surface having a plane orientation, according to the Miller indices, of; performing mechanical polishing on the main surface of the diamond crystal using a polishing wheel that is rotating in a rotation direction such that: the diamond crystal contacts the polishing wheel at contact positions (4a, 4b) on a curve that extends in the rotation direction of the polishing wheel, wherein a tangent contacts the curve that extends in the rotation direction of the polishing wheel at each contact position (4a, 4b); and the tangent having a tangent direction (4a1, 4b1) at each contact position (4a, 4b) within a range of ±10 degrees relative, according to the Miller indices, to a <110> direction of the diamond crystal, wherein the polishing wheel is moved relative to the main surface while kept in contact with the entire area of the main surface such that the angle formed between the tangent directions (4a1, 4b1) and, according to the Miller indices, the <110> direction of the diamond crystal at each of the contact positions with the polishing wheel, falls within the defined range of ±10°, thereby causing affected regions to appear such that the affected regions are parallel to a direction of a plane orientation of the diamond crystal and penetrate the diamond crystal onto the main surface, wherein the affected regions include dislocations; and performing chemical mechanical polishing on the main surface of the diamond crystal to remove the affected regions parallel to the direction of the plane orientation, thereby removing the affected regions from the main surface. preparing a diamond crystal having a main surface having a plane orientation, according to the Miller indices, of; performing mechanical polishing on the main surface of the diamond crystal using a polishing wheel that is rotating in a rotation direction such that: the diamond crystal contacts the polishing wheel at contact positions (4a, 4b) on a curve that extends in the rotation direction of the polishing wheel, wherein a tangent contacts the curve that extends in the rotation direction of the polishing wheel at each contact position (4a, 4b); and the tangent having a tangent direction (4a1, 4b1) at each contact position (4a, 4b) within a range of ±10 degrees relative, according to the Miller indices, to a <110> direction of the diamond crystal, wherein the polishing wheel is moved relative to the main surface while kept in contact with the entire area of the main surface such that the angle formed between the tangent directions (4a1, 4b1) and, according to the Miller indices, the <110> direction of the diamond crystal at each of the contact positions with the polishing wheel, falls within the defined range of ±10°, thereby causing affected regions to appear such that the affected regions are parallel to a direction of a plane orientation of the diamond crystal and penetrate the diamond crystal onto the main surface, wherein the affected regions include dislocations; and performing chemical mechanical polishing on the main surface of the diamond crystal to remove the affected regions parallel to the direction of the plane orientation, thereby removing the affected regions from the main surface. the diamond crystal contacts the polishing wheel at contact positions (4a, 4b) on a curve that extends in the rotation direction of the polishing wheel, wherein a tangent contacts the curve that extends in the rotation direction of the polishing wheel at each contact position (4a, 4b); and the tangent having a tangent direction (4a1, 4b1) at each contact position (4a, 4b) within a range of ±10 degrees relative, according to the Miller indices, to a <110> direction of the diamond crystal, wherein the polishing wheel is moved relative to the main surface while kept in contact with the entire area of the main surface such that the angle formed between the tangent directions (4a1, 4b1) and, according to the Miller indices, the <110> direction of the diamond crystal at each of the contact positions with the polishing wheel, falls within the defined range of ±10°, thereby causing affected regions to appear such that the affected regions are parallel to a direction of a plane orientation of the diamond crystal and penetrate the diamond crystal onto the main surface, wherein the affected regions include dislocations; and performing chemical mechanical polishing on the main surface of the diamond crystal to remove the affected regions parallel to the direction of the plane orientation, thereby removing the affected regions from the main surface.