iron flat plate is coated with diamond abrasive grain particle diameter of from 0 3 u03bcm to 1 u03bcm together with oil the sample tray on which the single crystal diamond substrate 1 1 is bonded is placed on the iron flat plate and is then fixed and then the iron flat plate is rotated for grinding after grinding has been performed for a fixed time period the inclined angle is measured by x ray diffraction measurement the above described grinding and the x ray diffraction measurement are repeated until the desired angle u03b1s is obtained in other words as shown in fig3 b the substrate is cut away until the surface orientation of the single crystal diamond substrate reaches a predetermined u03b1s by grinding the face inclined u03b1s u00b0 may be only the face on which a single crystal diamond thin film will be grown in the next process step described later next as shown in fig3 c crystal growth is performed on the single crystal diamond substrate 1 1 having the surface orientation angle u03b1s to form a single crystal diamond thin film 1 2 thereon this crystal growth of the thin film may be carried out by for example a microwave plasma cvd technique more specifically a methane gas and a hydrogen gas the flow ratio of methane gas is 1 are fed as a source gas into a reaction tube in which the single.