PURPOSE: To more exactly detect distortion by approximately calculating the coordinate values of picture elements in a distorted image from the coordinate values in a non-distorted image and determining the real coordinate values in the distorted image.

CONSTITUTION: An X-ray source 1 and an X-ray image amplifier 2 at a cross section synthesizer are mutually oppositely moved through linear routes to a lot of position to form X-ray images. The X-ray images provided at various radiation source positions are overlapped so as to form a slice image and the slice image of an object placed between the radiation source 1 and the X-ray image amplifier 2 is sharply imaged but all the other areas of an object are made nonsharp on the other hand. In this case, a test object G is arranged on a fulcram plane 3 in order to quantitatively discriminate the distortion. Then, overlapping can be executed only after the conversion of test images to non-distorted output images A and A'. The distortion corrected output images are coincident with the test object G excepting for a reduced scale.