It is an object of the present invention to provide a diamond substrate with high toughness, a large surface area, and high quality, for use in semiconductor materials, electronic components, optical components, and so forth, and a method for manufacturing this substrate.

A diamond polycrystalline film is laminated on the surface of a diamond monocrystalline substrate to create a diamond composite substrate. In said diamond composite substrate, it is preferable that the main face, which has the largest surface area of the diamond monocrystalline substrate, be the {100} plane, and the diamond polycrystalline film be laminated on the opposite face parallel to this face. The diamond monocrystalline substrate 3 may be made up of a plurality of diamond monocrystals having the same orientation of the main face, and these plurality of diamond monocrystals may be joined by a diamond crystal layer 4 to create a diamond composite substrate 2. The diamond monocrystals may also be used as seed crystals and diamond monocrystals provided by vapor phase synthesis on the surface thereof.