Provided is an X-ray irradiator which reduces the occurrence of discharge resulting from the difference in electric potential in the X-ray irradiator, and which concurrently achieves reduction in size and weight. In an X-ray irradiator (1), an X-ray tube (11) and a high-voltage generator (2) are installed inside a casing (18), and an insulation oil (13) is filled in the casing (18). The high-voltage generator (2) is configured by arranging and electrically connecting together multiple ring-shaped voltage amplifying units (21). An anode (14) and a cathode (15) of the X-ray tube (11) are fitted in and thus installed in hollow portions of the voltage amplifying units (21).