A radiation generating tube includes a vacuum envelope formed by an insulating tube, a cathode and an anode. At least one of the cathode and the anode is bonded to the insulating tube via a conductive bonding material disposed between bonded surfaces facing each other. The conductive bonding material is partially protruding from between the bonded surfaces to an outer peripheral surface or an inner peripheral surface of the insulating tube. A concave portion is formed on the outer peripheral surface or the inner peripheral surface of the insulating tube adjacent to a bonded surface on an insulating tube side, and a distal end of a conductive bonding material protruding from between bonded surfaces is accommodated in the concave portion.