The present invention provides a manufacturing method for a large-scale diamond substrate and a substrate produced by the method suitable for semiconductor lithography processing and large-scale optical parts, semiconductor materials, thermal-release substrate, semiconductor wafer processing, and back-feed devices, and others. The manufacturing method for a diamond substrate of the present invention comprises: the mounting step of preparing a substrate having a main face comprising a first region which is a concave and having a second region which surrounds the first region, and mounting, on the first region, a single crystalline diamond seed substrate having a plate thickness thicker than the concave depth of the first region; a connecting step of forming a CVD diamond layer from the single crystalline diamond seed substrate using a chemical vapor deposition, and mutually connecting by forming a CVD diamond layer on the second region at the same time; and a polishing step of polishing to substantially flatten both the CVD diamond layers on the single crystalline diamond seed substrate and on the second region by mechanically polishing.