In a radiography system, after a radiation dose, signal charges are accumulated in sensor pixels in an imaging area of a flat panel detector corresponding to the dosed amounts of radioactive rays on the pixels. The signal charges are read out from the pixels and converted into voltage signals representative of density levels of respective pixels of a radiographic image. Before reading the signal charges, a dose profile representative of distribution of the dosed amounts of radioactive rays is measured by leak currents from the pixels or bias currents through bias lines for applying a bias voltage to the pixels. Based on the contrast or difference between the maximum value and the minimum value in the dose profile, a gain of amplifiers for the voltage signals is determined such that the gain increases with decreasing contrast.