To solve the problem that it is difficult to measure a correct focal size owing to the blur of an image at the edge part of a pinhole in a shielding body since an X-ray is transmitted through the edge part when the X-ray focal size of an X-ray CT apparatus with highly strong X-ray energy is to be obtained in measurement by a pinhole method, i.e., a conventional method for measuring an X-ray source focal size.

The X-ray source focal size measuring instrument is provided with: an X-ray source 1 being an object to be measured; an imaging plate 2 arranged to face the X-ray source 1; an X-ray shielding body 3 arranged between the X-ray source 1 and the imaging plate 2; and a table 4 being a moving means for moving the shielding body 3 in a direction to be orthogonally crossed with a line which connects the X-ray source 1 to the imaging plate 2. The shielding body 3 has a thickness D for perfectly shielding the X-ray to be irradiated from the X-ray source 1. The pinhole 3a, which penetrates in a direction in parallel with the line for connecting the X-ray source 1 to the imaging plate 2, is formed in the shielding body 3.