1. An X-ray diagnosis apparatus, comprising: an X-ray tube configured to generate X-rays; an X-ray detector configured to detect the X-rays that have passed through a subject; and processing circuitry configured to calculate, for each X-ray image of a plurality of X-ray images that are acquired chronologically, an average of pixel values of the X-ray image, and a reference value, based on the pixel values of the X-ray image, and select, from the plurality of X-ray images, an X-ray image with a largest difference between the average and the reference value, an X-ray image with a highest ratio between the average and the reference value, or an X-ray image with a largest number of pixels, each representing a value equal to or larger than the average or the reference value.