An X-ray detector, used in an X-ray computerized tomograph systems, and method of making the same. The X-ray detector has a casing filled with gas for detecting X-rays, and flat signal electrode plates and bias electrode plates are disposed alternately at regular intervals within the casing to form a number of spatially partitioned detection chambers. When X-rays enter the detection chambers after being transmitted through an object under examination, the X-ray energy is converted into electric current flowing across the electrodes of the signal electrode plates, whereby the transmitted X-ray can be detected by an output circuit.Improvements are provided to enable the signal electrode plates and the bias electrode plates to be more densely packed into the casing, thereby to obtain higher resolution X-ray computerized tomograms. In one embodiment, the signal electrode plate comprises an electrically conductive plate or substrate, insulating layers on both sides of the substrate, and electrodes on the other sides of the insulating layers. In another embodiment, flat insulating plates are used to securely attach and suitably position alternately, the signal and bias plates. In a further embodiment, electrically conductive rubber cords are used to connect the respective signal and bias plates to a read out circuit. Cut-outs and insulating devices may be used on the signal and bias plates or in the flat plates, to suitably connect and prevent erroneous connection of the cord to other than the associated plates. In a further embodiment, the signal plates have protrusions and printed circuit boards are provided with suitable electrical circuit patterns for connecting the signal plates to the read-out circuit. In another embodiment, a jig comprising two oppositely disposed blocks having channels therein for holding the side edges of the bias and signal plates, is provided, with the blocks being electrically insulating and substantially X-ray transparent.