An apparatus for detecting X-rays and a driving method of the X-ray detecting apparatus. The driving method includes sampling a first data voltage corresponding to a rheobase voltage and a bias voltage, the rheobase voltage being generated by a current from a photodiode, sampling a second data voltage corresponding to the bias voltage after resetting the rheobase voltage, and resetting the rheobase voltage from the time that the sampling of the second data voltage is finished to the time that a corresponding frame is finished. An image delay generated when an X-ray motion picture is displayed may be minimized.