An X-ray computed tomography apparatus comprising: an electrocardiogram synchronous scanning means having: an X-ray source which generates X-rays; a high voltage generating unit which generates a high voltage to be applied the X-ray source; and an X-ray detector which detects X-rays transmitted through a subject to be examined to generate projection data, setting means for setting a heart rate fluctuation range determined based on a heart rate of the subject during a breathing exercise, the heart rate fluctuation range defining a shortest cardiac cycle and a longest cardiac cycle of the subject; wherein the means is in synchronization with an electrocardiography synchronized signal or an electrocardiography wave form signal of the examined subject; and a scanning controlling unit adapted to perform X-ray modulation control in which the X-rays are modulated from low output to high output when a predetermined period of time has elapsed from a R-wave pulse and a first time is reached, continued at high output state when a predetermined period of time has elapsed from the R-wave pulse and a second time is reached, and to repeat said X-ray modulation control for each cardiac cycle of the subject, characterised in that when the heart rate of the subject exceeds a predetermined upper heart rate, or when heart rate in the last cardiac cycle is higher than the heart rate in the immediately preceding cardiac cycle and difference between those heart rates exceeds a predetermined upper limit heart rate difference, the scanning controlling unit stops subsequent X-ray output modulation control to steadily maintain X-ray at the high output, wherein the first time is the time elapsed from the R-wave pulse corresponding to a first cardiac phase and determined on a basis of the shortest cardiac cycle, and the second time is the time elapsed from the R-wave pulse corresponding to a second cardiac phase and determined on a basis of the longest cardiac cycle. an electrocardiogram synchronous scanning means having: an X-ray source which generates X-rays; a high voltage generating unit which generates a high voltage to be applied the X-ray source; and an X-ray detector which detects X-rays transmitted through a subject to be examined to generate projection data, an X-ray source which generates X-rays; a high voltage generating unit which generates a high voltage to be applied the X-ray source; and an X-ray detector which detects X-rays transmitted through a subject to be examined to generate projection data, setting means for setting a heart rate fluctuation range determined based on a heart rate of the subject during a breathing exercise, the heart rate fluctuation range defining a shortest cardiac cycle and a longest cardiac cycle of the subject; wherein the means is in synchronization with an electrocardiography synchronized signal or an electrocardiography wave form signal of the examined subject; and a scanning controlling unit adapted to perform X-ray modulation control in which the X-rays are modulated from low output to high output when a predetermined period of time has elapsed from a R-wave pulse and a first time is reached, continued at high output state when a predetermined period of time has elapsed from the R-wave pulse and a second time is reached, and to repeat said X-ray modulation control for each cardiac cycle of the subject, characterised in that when the heart rate of the subject exceeds a predetermined upper heart rate, or when heart rate in the last cardiac cycle is higher than the heart rate in the immediately preceding cardiac cycle and difference between those heart rates exceeds a predetermined upper limit heart rate difference, the scanning controlling unit stops subsequent X-ray output modulation control to steadily maintain X-ray at the high output, wherein the first time is the time elapsed from the R-wave pulse corresponding to a first cardiac phase and determined on a basis of the shortest cardiac cycle, and the second time is the time elapsed from the R-wave pulse corresponding to a second cardiac phase and determined on a basis of the longest cardiac cycle.