1. A dynamic radiographing system comprising: a radiographing device which radiographs dynamically a heart part of a subject by plain radiography and generates X-ray images in a plurality of time phases, the radiographing device including an X-ray source for emitting an X-ray and a detector for detecting the X-ray emitted from the X-ray source; an image analysis device for calculating an evaluation value relating to a heart function using the X-ray images in the plurality of time phases; a display device; and a control device which allows the display device to display thereon information on the calculated evaluation value wherein the evaluation value includes an evaluation value of blood speed and wherein the image analysis device detects images of a pulsating part from the X-ray images in the plurality of time phases, and calculates a center of gravity of the pulsating part detected from each of the X-ray images in the plurality of time phases, and then calculates a traveling distance of the center of gravity per unit time as the evaluation value of blood speed.