A diamond smoothing method of irradiating a laser light onto a raised and recessed surface of a diamond, so as to smooth the raised and recessed surface, by ablation that is caused to occur in the diamond by irradiation of the laser light onto the raised and recessed surface. The method includes: a threshold-energy-density detecting step of irradiating the laser light onto the raised and recessed surface, and changing an irradiation energy density of the laser light, so as to detect a threshold energy density as a lower threshold value of the irradiation energy density that causes the ablation to occur; and a smoothing processing step of executing a smoothing processing by irradiating the laser light onto the raised and recessed surface with a smoothing irradiation energy density that is set to be within a range from 1 to 15 times as large as the threshold energy density.