An apparatus for generating X-ray may include: a plasma chamber; a magnet unit for applying a magnetic field to the plasma chamber, the magnet unit configured to allow the control of the magnitude of the minimum magnetic field in the plasma chamber without change in structure; a microwave generator for applying microwaves to the plasma chamber; a reaction gas injected into the plasma chamber for generating X-ray through electron cyclotron resonance by the magnetic field and the microwaves; a variable guide for focusing the generated X-ray; and a variable extractor for outputting the focused X-ray from the plasma chamber.