An X-ray diagnostic apparatus comprising: an X-ray tube configured to generate X-rays; an X-ray detector configured to detect X-rays generated by the X-ray tube and transmitted through a subject; an image generating unit configured to generate a plurality of time-series medical images of vasoganglion in a predetermined organ of the subject based on an output from the X-ray detector; a region setting unit configured to set a first upstream region and a first downstream region of a stenosis location in a first blood vessel in the vasoganglion on the medical images; a curve generating unit configured to generate a stenosis upstream curve indicating a change in pixel value in the time-series based on a plurality of pixel values included in the first upstream region, and generate a stenosis downstream curve indicating a change in pixel value in the time-series based on a plurality of pixel values included in the first downstream region; a stenosis index generating unit configured to generate a time-based stenosis index indicating a degree of stenosis in the first blood vessel based on time information of the stenosis upstream curve and time information of the stenosis downstream curve; and a display unit configured to display the time-based stenosis index, wherein the stenosis index generating unit is configured to generate the time-based stenosis index based on a first time from a start timing to a timing of a peak value on the stenosis upstream curve and a second time from a start timing to a peak value on the stenosis downstream curve, characterised in that the stenosis index generating unit is configured to generate a ratio of the first time to the second time as the time-based stenosis index. an X-ray tube configured to generate X-rays; an X-ray detector configured to detect X-rays generated by the X-ray tube and transmitted through a subject; an image generating unit configured to generate a plurality of time-series medical images of vasoganglion in a predetermined organ of the subject based on an output from the X-ray detector; a region setting unit configured to set a first upstream region and a first downstream region of a stenosis location in a first blood vessel in the vasoganglion on the medical images; a curve generating unit configured to generate a stenosis upstream curve indicating a change in pixel value in the time-series based on a plurality of pixel values included in the first upstream region, and generate a stenosis downstream curve indicating a change in pixel value in the time-series based on a plurality of pixel values included in the first downstream region; a stenosis index generating unit configured to generate a time-based stenosis index indicating a degree of stenosis in the first blood vessel based on time information of the stenosis upstream curve and time information of the stenosis downstream curve; and a display unit configured to display the time-based stenosis index, wherein the stenosis index generating unit is configured to generate the time-based stenosis index based on a first time from a start timing to a timing of a peak value on the stenosis upstream curve and a second time from a start timing to a peak value on the stenosis downstream curve, characterised in that the stenosis index generating unit is configured to generate a ratio of the first time to the second time as the time-based stenosis index.