A method for polishing a diamond crystal includes preparing a diamond crystal having a main surface having a plane orientation of (100). Mechanical polishing is performed on the main surface using a polishing wheel such that: a tangent contacts a curve extending in a rotation direction of the wheel and contacting a contact position between the diamond crystal and the wheel that is rotating; and a tangent direction of the tangent at the contact position is within a range of ±10 degrees relative to a direction of the diamond crystal, thereby causing an affected region to appear such that the affected region is parallel to a direction of a plane orientation (111) of the diamond crystal and penetrates the diamond crystal onto the main surface. Chemical mechanical polishing is performed on the main surface to remove the affected region, thereby removing the affected region from the main surface.