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 has passed through a subject; and processing circuitry configured to calculate, for each of a plurality of X-ray images that are acquired chronologically, an average of pixel values and a reference value based on the pixel values, and extract, from the plurality of X-ray images, an X-ray image with a relatively large difference between the average and the reference value, an X-ray image with a relatively large ratio between the average and the reference value, or an X-ray image with a relatively large number of pixels each representing a value equal to or larger than the average or the reference value.