A diamond joined body is a diamond joined body including a hard substrate and a polycrystalline diamond layer arranged on the hard substrate, wherein an area ratio of carbon grains in a region of the hard substrate is less than 0.03%, the region being a region enclosed by an interface between the hard substrate and the polycrystalline diamond layer and an imaginary line x in a cross section parallel to a normal direction of the interface, the imaginary line x being parallel to the interface on the hard substrate side and having a distance of 500 μm from the interface.