An X-ray computed tomography apparatus of this invention includes a first data detecting system which includes a first X-ray tube and a first X-ray detector and a second data detecting system which includes a second X-ray tube and a second X-ray detector. A reconstructing unit reconstructs image data on the basis of the data detected by at least one of the first and second data detecting systems. A monitoring unit monitors the first data detecting system. In a period during which the first data detecting system is normal, data acquisition is performed by both the first and second data detecting systems. In a period during which the first data detecting system is faulty, data acquisition is performed by the second data detecting system alone.