1. An X-ray diagnosis apparatus comprising: an image data generating unit ( 24 ) that generates X-ray images in a time sequence by detecting an X-ray that is radiated from an X-ray tube ( 12 ) and is transmitted through an subject; a feature-point-position detecting unit ( 26 a ) that detects a position of a feature point included in a predetermined target object from at least a first image and a second image included in the X-ray images generated in a time sequence by the image data generating unit ( 24 ); an application range determining unit ( 26 c ) that determines an application range of a smoothing filter in the first image and the second image based on a position of the feature point in the first image and a position of the feature point in the second image detected by the feature-point-position detecting unit ( 26 a ); and a filtering unit ( 26 d ) that performs a smoothing filter process by using the first image and the second image based on the application range of the smoothing filter determined by the application range determining unit ( 26 c ).