An X-ray CT apparatus includes an X-ray generating section for generating X-rays with a plurality of X-ray tube voltages; a data collecting section for collecting, synchronously with a data collection signal, X-ray projection data from X-rays with each X-ray tube voltage switched by an X-ray tube voltage switching signal; and a control section for controlling a timed moment for switching of the X-ray tube voltage and a timed moment for start of data collection so that the timed moment for start of data collection at each X-ray tube voltage in the data collection signal is delayed relative to the timed moment for switching of the X-ray tube voltage to that X-ray tube voltage in the X-ray tube voltage switching signal by .DELTA.t1, .DELTA.t2 depending upon imaging conditions.