A compact and low-cost electromagnetic wave generator in which X-rays having high intensity can be generated and the energy of generated X-rays can rapidly be switched. In an electromagnetic wave generator including a circular accelerator, a deflection electromagnet incorporated in the circular accelerator focuses injected and accelerated electrons. The circular accelerator produces stable closed electron orbits in respective regions with respective widths in the radial direction of the accelerator. The closed electron orbits are stable during injection and acceleration of electrons. A target is arranged across only some of the stable closed electron orbits so that a collision region, where a circulating electron beam collides with the target, and a non-collision region, where a circulating electron beam does not collide with the target, are produced. Through control of respective patterns of changes with time of the deflection magnetic field, a given electron closed orbit is shifted between the collision and the non-collision regions, thereby generating X-rays.