PROBLEM TO BE SOLVED: To provide a medical image diagnostic apparatus which comprises an X-ray tube having a grid and a variable focal size and can easily calibrate relation between the focal size and grid voltage, and a focal size calibration method.SOLUTION: A medical image diagnostic apparatus 10 comprises: an X-ray tube 13 an X-ray detection unit 14 a grid control unit 23 an X-ray optical unit 15 an image analysis unit 16 and a main control unit 21. The grid control unit 23 generates grid voltage according to set grid voltage and applies the grid voltage to a grid 32. The image analysis unit 16 outputs the size of a focal image which is formed on the X-ray detection unit 14 via the X-ray optical unit 15 while being irradiated with an X-ray emitted from the X-ray tube. The main control unit 21 outputs the set grid voltage to the grid control unit 23, calculates the focal size of the X-ray tube 13 on the basis of the size of the focal image formed according to the output set grid voltage, and generates information obtained by associating the calculated focal size with the set grid voltage output to the grid control unit 23.
