1. A three-dimensional X-ray CT apparatus, comprising: a CT imaging portion for continuously rotating relative angular positions of a subject and a measurement system to perform a CT imaging measurement for taking data on a plurality of transmission images for reconstructing a three-dimensional CT image of the subject; and an image reconstruction portion for reconstructing the three-dimensional CT image based on the data on the plurality of transmission images taken by the CT imaging portion, wherein during a period in which the CT imaging measurement is being performed, the image reconstruction portion reconstructs the three-dimensional CT image based on the already-taken data on the transmission images and displays the three-dimensional CT image before the CT imaging measurement is completed.