An initial image (the image of a slit plate 5 imaged when adjusted to an optimal focal diameter) is stored in a storage section 72 of an X-ray tube adjusting apparatus 7. An acquisition section 74 acquires a test image (the image of the slit plate 5 imaged at the time of adjusting the focal diameter). A presentation section 76 presents the initial image and an image representing the luminance on the initial image (showing a contrast .DELTA.a between a slit portion 764a and a residual area portion 766a in the initial image) and the test image and an image representing the luminance on the test image (showing a contrast .DELTA.b between a slit portion 764b and a residual area portion 766b in the initial image) simultaneously (in a comparable manner).