An X-ray inspection apparatus comprising: a conveyance unit configured to convey an object to be inspected; an X-ray radiation unit configured to radiate X-rays to the object conveyed by the conveyance unit; an X-ray detection unit (310, 320) configured to detect the X-rays radiated to the object conveyed by the conveyance unit; and an image processing unit configured to generate an X-ray transmission image of the object based on a detection signal output from the X-ray detection unit (310, 320) and carry out image processing on the X-ray transmission image; wherein the X-ray detection unit (310, 320) includes a plurality of direct conversion-type X-ray detection element arrays: disposed side-by-side in rows along a direction intersecting both a conveyance direction in which the object is conveyed by the conveyance unit and a radiation direction in which the X-rays are radiated by the X-ray radiation unit, and configured to convert X-rays of a first energy band into electric charge; characterized in that the image processing unit includes: an edge detection unit configured to carry out edge detection processing on the X-ray transmission image to generate an edge detected image, a horizontal direction gradation unit configured to carry out horizontal direction gradation processing along the conveyance direction on the edge detected image to generate a horizontal direction gradation image, and a synthesizing unit configured to synthesize the X-ray transmission image and the horizontal direction gradation image to generate a post-processing X-ray transmission image. a conveyance unit configured to convey an object to be inspected; an X-ray radiation unit configured to radiate X-rays to the object conveyed by the conveyance unit; an X-ray detection unit (310, 320) configured to detect the X-rays radiated to the object conveyed by the conveyance unit; and an image processing unit configured to generate an X-ray transmission image of the object based on a detection signal output from the X-ray detection unit (310, 320) and carry out image processing on the X-ray transmission image; wherein the X-ray detection unit (310, 320) includes a plurality of direct conversion-type X-ray detection element arrays: disposed side-by-side in rows along a direction intersecting both a conveyance direction in which the object is conveyed by the conveyance unit and a radiation direction in which the X-rays are radiated by the X-ray radiation unit, and configured to convert X-rays of a first energy band into electric charge; disposed side-by-side in rows along a direction intersecting both a conveyance direction in which the object is conveyed by the conveyance unit and a radiation direction in which the X-rays are radiated by the X-ray radiation unit, and configured to convert X-rays of a first energy band into electric charge; characterized in that the image processing unit includes: an edge detection unit configured to carry out edge detection processing on the X-ray transmission image to generate an edge detected image, a horizontal direction gradation unit configured to carry out horizontal direction gradation processing along the conveyance direction on the edge detected image to generate a horizontal direction gradation image, and a synthesizing unit configured to synthesize the X-ray transmission image and the horizontal direction gradation image to generate a post-processing X-ray transmission image. an edge detection unit configured to carry out edge detection processing on the X-ray transmission image to generate an edge detected image, a horizontal direction gradation unit configured to carry out horizontal direction gradation processing along the conveyance direction on the edge detected image to generate a horizontal direction gradation image, and a synthesizing unit configured to synthesize the X-ray transmission image and the horizontal direction gradation image to generate a post-processing X-ray transmission image.