An X-ray CT apparatus detects a predetermined characteristic wave from each electrocardiographic waveform acquired from an electrocardiograph, predicts intervals with which the characteristic wave appears based on appearance times of the detected predetermined characteristic wave in each electrocardiographic waveform, and determines an X-ray irradiation start time based on the predicted interval time. After an instruction to start collection of X-ray intensity distribution data is received, if an appearance interval of a detected predetermined characteristic wave is within an acceptable normal-heartbeat range, it is determined as a normal heartbeat. If the appearance interval of the detected predetermined characteristic wave is outside the acceptable normal-heartbeat range, it is determined as an abnormal heartbeat. During a period after receiving the instruction to start the collection until reaching a time of starting the collection of X-ray detection information, when determined as an abnormal heartbeat, an X-ray irradiation start time is recalculated, and then determined.