Provided is a radiation generating tube in which an insulating tube is prevented from being damaged by heat generation of a target or an electron emitting source during drive. Extending portions extending along an outer periphery of an insulating tube are provided to a cathode and an anode, respectively, and the insulating tube is joined to the extending portions to enhance the strength of joint portions. At the same time, the insulating tube can be deformed easily by setting a tube wall thickness of the insulating tube at a central portion in a longitudinal direction to be smaller than a tube wall thickness of an opening end, and thus the concentration of thermal stress on the joint portions caused by an increase in temperature of the cathode and the anode is alleviated.