1. An X-ray image diagnostic apparatus, comprising: an X-ray generation unit that generates X-rays according to X-ray conditions defining an output of X-rays; an X-ray detection unit that detects X-rays transmitted through an object and outputs a transmitted X-ray signal; an image generation unit that generates an X-ray image of the object on the basis of the transmitted X-ray signal; a histogram generation unit that generates a histogram showing a distribution of brightness values of the X-ray image; an object region detection unit that detects an object region, which is a region where the object is imaged, in the X-ray image on the basis of the histogram; and an X-ray condition determination unit that determines the X-ray conditions so that a brightness value representing the object region approaches a target brightness value of the object region set in advance, wherein the X-ray generation unit generates the X-rays according to the determined X-ray conditions.