A radiation generating apparatus includes a radiation generation tube including an electron emitting source having an electron emitting member, a transmission type target, a tubular backward shielding member having an electron passing hole facing the target layer at one end, located at the electron emitting source side of the transmission type target, and connected to the periphery of the base member. The radiation generating apparatus further includes a collimator having an opening for defining an angle for extracting the radiation at the opposite side of the electron emitting source side of the transmission type target, and an adjusting device connected to the collimator, and configured to vary an opening diameter of the opening, wherein the target layer has a portion separated from a connection portion of the base member and the backward shielding member at the periphery.
