A radiation generating apparatus includes a radiation generating unit and a diaphragm unit that functions as a projector-collimator configured to simulate a radiation field with a visible-light field. The diaphragm unit includes a light source configured to generate visible light, an optical lens configured to control a state of diffusion of the visible light emitted from the light source, and field-limiting blades. The light source and the optical lens are provided between a radiation emission window and the field-limiting blades. The light source is movable into and retractable from a path of radiation generated by the radiation generating unit.