A method for manufacturing a single crystal diamond in which vapor phase synthetic single crystal diamond is additionally deposited on a single crystal diamond seed substrate obtained by vapor phase synthesis, includes a step of measuring flatness of the seed substrate, a step of determining whether or not to flatten the seed substrate based on the measurement result of the flatness, and any one of the following two steps of a step of additionally depositing the vapor phase synthetic single crystal diamond after flattening the seed substrate for which the flattening is necessary based on the determination and a step of additionally depositing the vapor phase synthetic single crystal diamond without flattening the seed substrate for which the flattening is not necessary based on the determination.