A diagnostic x-ray system includes a CT-image projection unit obtains a third perfusive-flow image by projecting a second perfusive-flow image obtained by computed tomography in the same direction as a projection direction in which an X-ray is projected toward a test subject to obtain a first perfusive-flow image. A positioning unit obtains positioning information by performing positioning of the first perfusive-flow image and the third perfusive-flow image. A perfusive-flow index calculation unit calculates a first perfusive-flow index from the first perfusive-flow image, and calculates a second perfusive-flow index from the third perfusive-flow image. A comparative-index calculation unit calculates a comparative index based on the first perfusive-flow index and the second perfusive-flow index, by comparing the first perfusive-flow index with the second perfusive-flow index, based on the positioning information obtained by the positioning unit.