1. An X-ray diagnosis apparatus comprising: an X ray source which irradiates X ray to an object to be examined in a plurality of times with a predetermined time interval; an X ray flat panel detector which detects X ray each time transmitted through the subject and outputs the same as image data; a correction table which stores a characteristic of X ray dose versus output signal height of the X ray flat panel detector with respect to a first, second, third,. .. , nth (n is an integer more than 1) image data output from the X ray flat panel detector; and a processor which executes subtraction process of at least the second image data value the first image data value, the third image data value—the first image data value,. .. , the nth image data value—the first image data value, wherein the subtraction process in the processor is executed after matching one of the characteristic of X ray dose versus output signal height of the image data to be subtracted and the characteristic of X ray dose versus output signal height of the image data to subtract to the other.