A radiation generator includes: a radiation tube configured to generate radiation by emitting electrons from a cathode through a grid to a target; a grid-voltage generating unit configured to apply an extraction voltage to the grid in response to an external request for radiation output; a cut-off voltage generating unit configured to generate a cut-off voltage applied to the grid so as to lower the potential of the grid relative to the potential of the cathode when there is no external request for the radiation output; and a detection unit configured to detect a decrease in the cut-off voltage, wherein the target is not irradiated by the electrons when the decrease in the cut-off voltage is detected.