An image processor executes an image processing under an appropriate condition. The image possessor comprises an important area detecting section (120) for detecting a bone portion or soft portion as an important area included in an X-ray image, a weight image creating section (130) for creating a weight image for imparting a predetermined weight to the pixels of the important area, histogram computing section (140) for multiplying the weight of the weight image by the pixel value of the X-ray image and computing a weighted histogram from the results of the multiplication, evaluating section (150) for evaluating the weighted histogram with a predetermined evaluation function and computing a shift value at which the evaluation result takes on a maximum value, image processing condition determining section (160) for so determining an image processing condition so as to obtain a predetermined processing result from the pixel value of the X-ray image corresponding to the maximum value of the evaluation function for obtaining a shift value, and image processing section (170) for executing an image processing under the image processing condition.