An X-ray CT apparatus includes an X-ray generator and an X-ray detector. The X-ray generator and the X-ray detector rotate about a subject. A controller controls the timing of exposing X-rays from the X-ray generator. The controller controls the collection timing for the projection data used by the X-ray detector and causes the X-ray detector to continuously collect the projection data at the specified collection timing. The controller causes additional information representing the state of exposure of the X-rays to be added to each projection data that has been collected by the X-ray detector at said specified collection timing.