An ion beam apparatus includes a source part generating plasma therein, a process part in which a process using an ion beam is performed, and a slit structure provided between the source part and the process part and extracting the ion beam from the plasma. The slit structure includes at least one electrode structure. The electrode structure has a slit penetrating the electrode structure and extending in a first direction. The ion beam is irradiated onto a substrate at an incident angle through the slit. The incident angle of the ion beam is adjusted by rotating the electrode structure on a rotation axis parallel to the first direction.