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 structured for fixing the subject along center of rotation 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 the CT imaging measurement comprises taking data on a plurality of transmission images for a primary measurement, and taking data on a plurality of transmission images for a secondary measurement, which is measured in a rotation plane common to the primary measurement in succession to the primary measurement, the data on the plurality of transmission images for the primary measurement being data necessary for the image reconstruction portion to perform 180° image reconstruction or 360° image reconstruction, and wherein the image reconstruction portion is configured to: reconstruct an initial three-dimensional CT image based on the data on the plurality of transmission images for the primary measurement, and display the initial three-dimensional CT image after the secondary measurement is started; reconstruct a last three-dimensional CT image based on both the data on the plurality of transmission images for the primary measurement and the data on the plurality of transmission images for the secondary measurement, and display the last three-dimensional CT image, wherein the image reconstruction portion comprises: an input image data generating section for generating data on a plurality of input images based on the data on the plurality of transmission images taken by the CT imaging portion; and a reconstruction processing section for subjecting the data on the plurality of input images to reconstruction processing to generate the three-dimensional CT image, wherein in parallel to the primary measurement, the input image data generating section generates the data on the plurality of input images by using at least a part of the data on the plurality of transmission images for the primary measurement, and the reconstruction processing section subjects the data on the plurality of input images to the reconstruction processing to generate the initial three-dimensional CT image, and wherein in parallel to the secondary measurement, the input image data generating section generates the data on the plurality of input images by using at least a part of the data on the plurality of transmission images for the secondary measurement, and the reconstruction processing section subjects the data on the plurality of input images to the reconstruction processing to generate the last three-dimensional CT image, and wherein when the part of the data on the plurality of transmission images for the secondary measurement comprises data on a plurality of transmission images from the same angular position of a gantry, the input image data generating section weights the data on the plurality of transmission images in accordance with a rotation speed of the gantry and combines the weighted data on the plurality of transmission images to generate data on an input image at the angular position.