The objective of the present invention is to effectively improve an image lag phenomenon of a direct conversion detector. The present invention provides an X-ray detector comprising: a lower electrode, formed on a substrate, to which a first driving voltage V1 is applied; an auxiliary electrode, around the lower electrode, to which a third driving voltage V3 is applied; a photoconductive layer formed on the lower electrode and the auxiliary electrode; and an upper electrode, formed on the photoconductive layer, to which a second driving voltage V2 is applied, wherein the third driving voltage V3, right after the radiation of the X-rays is off, is a reverse bias voltage.