There is provided a method of reliably preparing a diamond semiconductor by irradiating diamond with a corpuscular ray. In this method, when a diamond substrate is irradiated with a corpuscular ray, the diamond substrate is maintained at a temperature of 300° C. to 2000° C., the angle of the surface of the diamond substrate irradiated is set within −20° to +20° to the (001) crystal plane of the diamond substrate, and the angle of the direction of the corpuscular ray is set within −20° to +20° to the crystal orientation of the diamond substrate. Preferably, the direction of the corpuscular ray forms an angle of 3° to 10° with the crystal orientation.