An X-ray imaging apparatus comprising: an X-ray source configured to: generate X-rays; and irradiate the X-rays; a photographing device installed in the X-ray source, the photographing device being configured to photograph a marker image; and a controller configured to: output, when none of a plurality of markers (M1 to M8) constituting a multi-marker array formed in a front surface of an X-ray detector is detected from the marker image, a notice informing that none of the plurality of markers (M1-M8) is detected; and detect a single marker (MS) that is attached to, coupled with or located next to the X-ray detector and that has features distinguished from those of the multi-marker array from another marker image photographed after the notice informing that none of the plurality of markers (M1-M8) is detected is output; and to determine a pose of the X-ray detector based on the detected marker. an X-ray source configured to: generate X-rays; and irradiate the X-rays; a photographing device installed in the X-ray source, the photographing device being configured to photograph a marker image; and a controller configured to: output, when none of a plurality of markers (M1 to M8) constituting a multi-marker array formed in a front surface of an X-ray detector is detected from the marker image, a notice informing that none of the plurality of markers (M1-M8) is detected; and detect a single marker (MS) that is attached to, coupled with or located next to the X-ray detector and that has features distinguished from those of the multi-marker array from another marker image photographed after the notice informing that none of the plurality of markers (M1-M8) is detected is output; and to determine a pose of the X-ray detector based on the detected marker. generate X-rays; and irradiate the X-rays; a photographing device installed in the X-ray source, the photographing device being configured to photograph a marker image; and a controller configured to: output, when none of a plurality of markers (M1 to M8) constituting a multi-marker array formed in a front surface of an X-ray detector is detected from the marker image, a notice informing that none of the plurality of markers (M1-M8) is detected; and detect a single marker (MS) that is attached to, coupled with or located next to the X-ray detector and that has features distinguished from those of the multi-marker array from another marker image photographed after the notice informing that none of the plurality of markers (M1-M8) is detected is output; and to determine a pose of the X-ray detector based on the detected marker. output, when none of a plurality of markers (M1 to M8) constituting a multi-marker array formed in a front surface of an X-ray detector is detected from the marker image, a notice informing that none of the plurality of markers (M1-M8) is detected; and detect a single marker (MS) that is attached to, coupled with or located next to the X-ray detector and that has features distinguished from those of the multi-marker array from another marker image photographed after the notice informing that none of the plurality of markers (M1-M8) is detected is output; and to determine a pose of the X-ray detector based on the detected marker. <EOS>