A cooler that circulates cooling fluid to cool an X-ray tube, and that comprises a circulation unit that has a first end unit and a second end unit, and an expansion mechanism that has a vessel and a bellows that divides the inside of the vessel into a first chamber and a second chamber, wherein the first end unit and the second end unit are detachably coupled to the X-ray tube and are closed when being uncoupled from the X-ray tube, and the expansion mechanism can be switched between a first state and a second state.