1. An X-ray image diagnostic apparatus, comprising: an X-ray generator that generates X-rays according to X-ray conditions defining an output of X-rays; an X-ray detector that detects X-rays transmitted through an object and outputs a transmitted X-ray signal; an image generation calculator that generates an X-ray image of the object on the basis of the transmitted X-ray signal; a histogram generation calculator that generates a histogram showing a distribution of brightness values of the X-ray image; an object region detection calculator 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 calculator 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; a feedback brightness value calculator that sets, as a reference brightness value, a brightness value when a ratio of the total number of pixels summed up from a minimum brightness value in the histogram to the total number of pixels of the histogram reaches a second threshold value set in advance and calculates, as a feedback brightness value, a value obtained by dividing a sum of values, which are obtained by multiplying respective brightness values from the minimum brightness value to the reference brightness value by the number of pixels of the corresponding brightness value, by a sum of the number of pixels from the minimum brightness value to the reference brightness value; wherein the X-ray generator generates the X-rays according to the determined X-ray conditions; and wherein the X-ray condition determination calculator determines the X-ray conditions so that the feedback brightness value matches the target brightness value.