1. An X-ray diagnosis apparatus comprising: a substantially C-shaped arm; a support mechanism configured to rotatably support the substantially C-shaped arm; a rotation driving unit configured to drive rotation of the substantially C-shaped arm; an X-ray tube mounted on the substantially C-shaped arm; an X-ray detector mounted on the substantially C-shaped arm so as to face the X-ray tube; a control unit configured to control the X-ray tube and the X-ray detector to make the X-ray tube continuously or intermittently generate X-rays during rotation of the substantially C-shaped arm and make the X-ray detector repeat X-ray detection; a storage unit configured to store a plurality of two-dimensional images generated under the control of the control unit; a first reconstruction processing unit configured to reconstruct a plurality of first three-dimensional images respectively corresponding to a plurality of cardiac phases based on the plurality of acquired two-dimensional images; a positional shift amount calculation unit configured to calculate a plurality of positional shift amounts respectively corresponding to the plurality of cardiac phases based on the plurality of first three-dimensional images; and a second reconstruction processing unit configured to reconstruct a single second three-dimensional image with a reduced positional shift based on the plurality of acquired two-dimensional images by using the calculated positional shift amounts.