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; and processing circuitry 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 in a plurality of images taken by the first image taking device, use information about the couchtop positions estimated for performing a position correcting process on the images obtained by the image taking devices, reconstruct the images taken by the second image taking device by using an attenuation map based on the images that have been taken by the first image taking device and have been corrected, and correct positional gaps of the reconstructed images with respect to the images that have been taken by the first image taking device.