A power supply apparatus for a detector of this invention provides a plurality of power supplies for the detector for detecting light or radiation； and includes a power output device capable of individually outputting the plurality of power supplies to be provided for the detector, a detection device for detecting a power supply state of a utility power source, a control device for carrying out an abnormal stopping process to stop the power supplies at the power output device in a predetermined order upon determination based on a result of detection by the detection device that an abnormality has occurred, and an electric storage device for supplying electric power to the control device, when the control device determines that an abnormality has occurred, to permit the control device to carry out the abnormal, stopping process. Thus, without having a large-scale uninterruptible power supply system, the detector can be protected even when the electric power supply state from the utility power source becomes abnormal.
