1. An image diagnosis apparatus comprising: a first image taking device configured to take an image of a patient placed on a couchtop by using an X-ray emission; a second image taking device configured to take images in positions by moving an image taking position of the patient by a predetermined distance at a time, along a body-axis direction; an estimating unit configured to estimate a couchtop position for each of the image taking positions of the second image taking device, based on information about warping of the couchtop of the first image taking device; and a position correcting unit configured to use information about the couchtop positions estimated by the estimating unit for performing a position correcting process on the images obtained by the image taking devices.