An X-ray diagnostic apparatus having an X-ray emitter configured to emit X-rays to a subject; an X-ray detector configured to detect X-rays transmitting through the subject; and a grid disposed on an X-ray incidence side of the X-ray detector to remove scattered X-rays, and configured to obtain an image in accordance with output from the X-ray detector, the X-ray diagnostic apparatus comprising: a peak frequency detector configured to detect a peak frequency of a first harmonic in a moiré pattern of the grid appearing in the image; a first and second harmonic remover configured to remove the first harmonic and a second harmonic in the moiré pattern from the image in accordance with the peak frequency of the first harmonic to obtain a first and second harmonic removed image; a third harmonic extracting-filter generating unit configured to calculate a peak frequency of a third harmonic in the moiré pattern in accordance with the peak frequency of the first harmonic to generate a third harmonic extracting filter to extract the third harmonic; a third harmonic extracting unit configured to extract the third harmonic from the first and second harmonic removed image based on the third harmonic extracting filter; a third harmonic subtracting unit configured to subtract the extracted third harmonic from the first and second harmonic removed image to obtain a third harmonic removed image; characterised in that the x-ray diagnostic apparatus further comprises a flatness calculating unit configured to calculate flatness in the first and second harmonic removed image to obtain flatness information; and an output selecting unit configured to output the third harmonic removed image within a flat region of the flatness information, and output the first and second harmonic removed image within a region out of the flat region of the flatness information. a peak frequency detector configured to detect a peak frequency of a first harmonic in a moiré pattern of the grid appearing in the image; a first and second harmonic remover configured to remove the first harmonic and a second harmonic in the moiré pattern from the image in accordance with the peak frequency of the first harmonic to obtain a first and second harmonic removed image; a third harmonic extracting-filter generating unit configured to calculate a peak frequency of a third harmonic in the moiré pattern in accordance with the peak frequency of the first harmonic to generate a third harmonic extracting filter to extract the third harmonic; a third harmonic extracting unit configured to extract the third harmonic from the first and second harmonic removed image based on the third harmonic extracting filter; a third harmonic subtracting unit configured to subtract the extracted third harmonic from the first and second harmonic removed image to obtain a third harmonic removed image; characterised in that the x-ray diagnostic apparatus further comprises a flatness calculating unit configured to calculate flatness in the first and second harmonic removed image to obtain flatness information; and an output selecting unit configured to output the third harmonic removed image within a flat region of the flatness information, and output the first and second harmonic removed image within a region out of the flat region of the flatness information.