A method for producing a single crystals by preferential epitaxial growth of {100} face, comprising the steps of (1) growing the crystal on a single crystal {100} substrate; (2) forming on the side of the grown crystal a surface parallel to a {100} face different from the {100}face in the growth direction, and (3) growing the crystal on the formed {100} surface; and the steps (2) and (3) being performed once or more than once. A method for producing a single-crystal diamond using a metallic holder for the single-crystal diamond having a crystal holding portion which is raised above an outer peripheral portion of the holder, is part from the outer peripheral portion of the holder, and has a recessed shape. The methods enable the production of a large single-crystal diamond in a comparatively short time at low cost.