The present invention provides a method for producing a large substrate of single-crystal diamond, including the steps of preparing a plurality of single-crystal diamond layers separated form an identical parent substrate, placing the single-crystal diamond layers in a mosaic pattern on a flat support, and growing a single-crystal diamond by a vapor-phase synthesis method on faces of the single-crystal diamond layers where they have been separated from the parent substrate. According to the method of the invention, a mosaic single-crystal diamond having a large area and good quality can be produced relatively easily.