An X-ray tube which is high in brightness and high in resolution, and can withstand continuous long-time use, that is, it can withstand a high heat load. An X-ray target and an X-ray tube having the X-ray target include an X-ray generating metal layer having an average crystal grain diameter not larger than 30 &mgr;m on the surface of a base plate in the X-ray irradiated side. The X-ray tube has a small focus point and can withstand a high input load. A CT apparatus using the X-ray tube can provide a high resolution and a high definition image.