PURPOSE: An X ray detector and a manufacturing method thereof are provided to increase a fill factor by forming a lower electrode unit made of transparent conductive materials on a drain electrode. CONSTITUTION: A PIN diode(DI) includes a lower electrode unit which is electrically connected to a drain electrode of a thin film transistor and is formed on the upper side of the drain electrode to cover the drain electrode of the thin film transistor, a photo conversion semiconductor arranged on the lower electrode unit, and an upper electrode unit formed on the photo conversion semiconductor. A diode protection layer(150) covers the PIN diode and includes a bias contact hole to expose the part of the upper side of the PIN diode. The part of the upper side of the PIN diode is exposed through the bias contact hole. A bias wiring(160) is electrically contacted with the upper side of the PIN diode through the bias contact hole. An optical wavelength transformer(190) transforms the X ray into the wavelength which is absorbed in the PIN diode.