1. An X-ray diagnostic imaging method for radiographing a predetermined area of a subject with a rotational motion of a single C-arm so as to acquire image data of a contrast image including a figure of a contrast medium flowing in an affected part, and for displaying and retaining the image data of the contrast image, comprising: acquiring a mask image of the predetermined area; injecting a contrast medium into the affected part after acquiring the mask image; starting a first radiograph of the predetermined area for a first contrast image when a time reaches an arterial phase after the injecting of the contrast medium into the affected part while rotating the C-arm in a first rotational direction; and starting a second radiograph of the predetermined area for a second contrast image after waiting for a time for the contrast medium to reach a venous phase after the arterial phase, while rotating the C-arm in a second rotational direction opposite to the first rotational direction.